Easy vbench testing of HEAD against a known-good baseline
I hope to avoid more performance regressions in the future, in my own PR's
if no one else's... 

Her'es some example output:

```
groupby_apply_dict_return                 33.20      33.26  1.00
frame_reindex_axis1                        2.32       2.32  1.00
frame_fillna_inplace                      18.34      18.31  1.00
frame_ctor_nested_dict_int64             106.89     106.40  1.00
index_int64_union                         94.21      93.68  1.01
write_csv_standard                       309.77     307.62  1.01
groupby_multi_different_functions         16.72      16.60  1.01
frame_ctor_nested_dict                    75.67      74.62  1.01
groupby_multi_python                      54.00      53.18  1.02
frame_to_csv                             356.20     347.55  1.02
groupby_multi_cython                      21.21      20.63  1.03
groupby_indices                            9.22       8.95  1.03
frame_ctor_list_of_dict                   95.60      91.88  1.04
series_ctor_from_dict                      2.84       2.65  1.07
sort_level_zero                            6.46       6.00  1.08
groupby_simple_compress_timing            70.36      62.56  1.12

Columns: test_name | head_time [ms] | baseline_time [ms] | ratio

- a Ratio of 1.30 means HEAD is 30% slower then the Baseline.

Head [951d13b] : ENH: Add test_perf.sh, make vbench dead easy.
Baseline [bdbca8e] : BUG: fix borked data-copying iteritems performance affecting DataFrame and SparseDataFrame. close #2273


*** Results were also written to the logfile at '/home/user1/src/pandas/vb_suite.log'
```

benchmarks for which HEAD takes less then 1ms are dropped.
## Remaining issues

1) vb_suite/join_merge.py contains two tests named 'join_dataframe_index_single_key_bigger'
, easy enough to fix, but that might break existing dbs, i'm not sure, so I left it alone.

2) Runner seems to choke on refs which are not an ancestor of the current HEAD
this looks like a shortcoming in vbench.GitRepo.
example trace:

``` python
Traceback (most recent call last):
  File "test_perf.py", line 134, in <module>
    main()
  File "test_perf.py", line 94, in main
    runner._run_and_write_results(h_baseline)
  File "/usr/local/lib/python2.7/dist-packages/vbench/runner.py", line 87, in _run_and_write_results
    n_active_benchmarks, results = self._run_revision(rev)
  File "/usr/local/lib/python2.7/dist-packages/vbench/runner.py", line 119, in _run_revision
    need_to_run = self._get_benchmarks_for_rev(rev)
  File "/usr/local/lib/python2.7/dist-packages/vbench/runner.py", line 173, in _get_benchmarks_for_rev
    timestamp = self.repo.timestamps[rev]
  File "/home/user1/src/pandas/pandas/core/series.py", line 470, in __getitem__
    return self.index.get_value(self, key)
  File "/home/user1/src/pandas/pandas/core/index.py", line 692, in get_value
    raise e1
KeyError: 'bdbca8e'
```

3) redirecting stdout/stderr does no work, maybe because os.system is used
under the covers. It frustrated my efforts funnel the crud into a log file,
and keep the output readable.

4) it would be nice if vbench.db.BenchmarkDB.get_ref_result() returned a fully
 labeled DataFrame, rather then just a partial dict (test names are not included)

Something like:

``` python
    def get_rev_full_results(self, revision):
        tab = self._results
        bench = self._benchmarks

        stmt = sql.select([tab.c.timestamp, tab.c.revision, tab.c.ncalls,
                           tab.c.timing, tab.c.traceback, tab.c.description,
                           tab.c.checksum,bench.c.name],
                           sql.and_(tab.c.revision == revision,tab.c.checksum == bench.c.checksum))
        results = self.conn.execute(stmt)

        df = _sqa_to_frame(results).set_index('name')
        return df.sort_index()
```
