SCR: add a script for tracking down all commits touching a named method

New script to get a change hirsoty for a functions across the
code base.

Initially, I hope this well help define "golden" commits for the various
adhoc versions of pickled pandas objects, so we can finish off #3310 
, get robust back-compat testing and feel easier about touching
the serialization code (which always scares me a little, pesonally).
That's also prep for #3297, and the changes jeff suggested there.

Example usage (check out the --help for filtering):
```
λ cd /tmp/pandas/pandas
λ ~/src/pandas/scripts/find_commits_touching_func.py __getstate__ 

These commits touched the __getstate__ method in these files on these dates:

138b3ef6 2010-01-05 * Cleaned up and sped up DataMatrix and DataFram ...  core/frame.py    
f49a3719 2010-06-04 code reorg in Series, miscellaneous groupby refa ...  core/frame.py    
9bddd0f4 2010-12-11 working toward consistent column ordering in Dat ...  core/frame.py    
c35a39de 2011-06-12 bit the bullet and did the big merge. now time t ...  core/frame.py    
739f8a1f 2011-06-09 picklability, column reordering issues, more ref ...  core/internals.py
3133a6b0 2011-06-12 need some changes to block internals, getting st ...  core/internals.py
1ef60ee0 2011-06-12 unit tests pass...winning!                            core/internals.py
32b9481d 2011-06-21 mixed-type fixes in DataFrame statistical methods     core/internals.py
4833ea9d 2011-06-30 biting the bullet, working to make generic n-dim ...  core/internals.py
80b91769 2011-08-15 ENH: support picklability of Index subclasses         core/internals.py
0f6d8b43 2009-09-01 latest edits, miscellaneous cleanup and bug fixe ...  core/matrix.py   
138b3ef6 2010-01-05 * Cleaned up and sped up DataMatrix and DataFram ...  core/matrix.py   
3ef46c24 2010-04-12 code rearrangement                                    core/matrix.py   
739f8a1f 2011-06-09 picklability, column reordering issues, more ref ...  core/matrix.py   
6fd65acd 2009-11-20 merging latest developments, in particular LongP ...  core/panel.py    
bf621508 2010-01-09 fleshing out WidePanel functionality. miscellane ...  core/panel.py    
00494462 2010-06-21 committing tests and changes in broken state, to ...  core/panel.py    
9eba5770 2011-06-30 removed usages of indexMap, misc panel stuff          core/panel.py    
8b4d2084 2011-05-03 skeleton of sparse WidePanel, unit tests and stu ...  core/sparse.py   
4218857a 2011-05-13 correct pickling of sparse frame/panel. setitem/ ...  core/sparse.py   
b3c12b15 2011-12-10 REF: broke apart sparse.py, created sparse subpackage sparse/frame.py  
a5f773bd 2012-06-04 BUG: cast other datetime64 units to nanos from I ...  sparse/frame.py  
b3c12b15 2011-12-10 REF: broke apart sparse.py, created sparse subpackage sparse/panel.py  
5fda0afc 2012-04-04 ENH: non-unique index support. much more testing ...  sparse/panel.py  

```