CLN: enforce four-space indentation
Many files:

```
['./bench/bench_join_panel.py',
 './pandas/sandbox/qtpandas.py',
 './pandas/tests/test_common.py',
 './pandas/tests/test_frame.py',
 './pandas/tests/test_generic.py',
 './pandas/core/series.py',
 './pandas/core/format.py',
 './pandas/core/internals.py',
 './pandas/core/indexing.py',
 './pandas/io/tests/test_sql.py',
 './pandas/tseries/converter.py',
 './setup.py',
 './doc/make.py',
 './doc/sphinxext/numpydoc/docscrape.py',
 './doc/sphinxext/ipython_sphinxext/ipython_directive.py']
```

have indentation which isn't four-space.  Sometimes it's consistently eight spaces:

```
def create_panels_append(cls, panels):
        """ return an append list of panels """
        panels = [a for a in panels if a is not None]
        # corner cases
```

and sometimes it's more inconsistent (5 here, but not elsewhere):

```
        with ensure_clean(pname) as path:

             self.frame['A'][:5] = nan
```

so that pylint spits out complaints like

```
W:5577,0: Bad indentation. Found 23 spaces, expected 20
```

There's also at least one rogue tab in `testing.py`:

```
    def assert_numpy_array_equal(self, np_array, assert_equal):
        if np.array_equal(np_array, assert_equal):
                return
        raise AssertionError('{0} is not equal to {1}.'.format(np_array, assert$
```

Easily fixed, although I wonder if it should be hooked to avoid it happening again.
