Potential bug resulting in MemoryError
I'm performing the following set of consecutive `merge` operations in pandas on a few small tables (100k rows by ~15 columns) that are read from file using `read_table`. Although the tables are small and I run it on a machine with over 60 GB of RAM, I consistently get `MemoryError` from the `merge` operation, and the traceback points to:

```
  File "/home/yarden/.local/lib/python2.6/site-packages/pandas-0.9.0-py2.6-linux-x86_64.egg/pandas/tools/merge.py", line 437, in _get_join_indexers
    return join_func(left_group_key, right_group_key, max_groups)
  File "join.pyx", line 105, in pandas.lib.left_outer_join (pandas/src/tseries.c:118232)
  File "join.pyx", line
```

The following zip contains the short script (`make_table.py`) and the necessary text files to reproduce the bugs:

http://genes.mit.edu/burgelab/yarden/merge_error_pandas.zip

To generate the error, simply run: 

`
python make_table.py
`

which for me yields the output:

```
Merging known to Ensembl...
  - Merge took 0.48 secs
Merging kgXref...
Traceback (most recent call last):
  File "make_table.py", line 137, in <module>
    main()
  File "make_table.py", line 124, in main
    right_on=["kgID"])
  File "/home/yarden/.local/lib/python2.6/site-packages/pandas-0.9.0-py2.6-linux-x86_64.egg/pandas/tools/merge.py", line 33, in merge
    return op.get_result()
  File "/home/yarden/.local/lib/python2.6/site-packages/pandas-0.9.0-py2.6-linux-x86_64.egg/pandas/tools/merge.py", line 180, in get_result
    join_index, left_indexer, right_indexer = self._get_join_info()
  File "/home/yarden/.local/lib/python2.6/site-packages/pandas-0.9.0-py2.6-linux-x86_64.egg/pandas/tools/merge.py", line 255, in _get_join_info
    sort=self.sort, how=self.how)
  File "/home/yarden/.local/lib/python2.6/site-packages/pandas-0.9.0-py2.6-linux-x86_64.egg/pandas/tools/merge.py", line 437, in _get_join_indexers
    return join_func(left_group_key, right_group_key, max_groups)
  File "join.pyx", line 105, in pandas.lib.left_outer_join (pandas/src/tseries.c:118232)
  File "join.pyx", line 190, in pandas.lib._get_result_indexer (pandas/src/tseries.c:119829)
MemoryError
```

I'm happy to provide any information that might be needed and/or test code that might fix this. Any insights on what might be causing this will be greatly appreciated. 
