ENH: Add wide_to_long convenience function
closes #4920

I thought I submitted this PR long ago.

looks fine, can you add a mention in release notes / v0.13.0 (example if you want)...thxs
 does this close an issue?
 As a remark from somebody who isn't familiar with the function(ality) and is just looking at the docstring, it is not really clear for me what it does. What is meant with wide panel/long format? What is a subobservation? What are stub names? Terminology that I don't really have encountered yet (in pandas).
In first sight, it seems a little bit specific (or the terminology). But is is possible it comes from STATA? (I searched for "stub names" on the internet, and the only relevant thing I found was something from reshape in STATA).
 @jseabold - you proposed it here - #4920 :)
 We'd discussed putting this as a keyword arg to melt in #4920 - thoughts on that?
 @jorisvandenbossche Yes, it's inspired by stata, but wide format panel data is fairly common in longitudinal datasets in economics. There are many stack overflow questions on how to do this in pandas.

Stub names are what I'm calling the common roots of the variable names. Stub because it's the shorter part of the variable name that you want. E.,g. for "A\d*", it's "A". See the example given in the docstring.

@jratner I guess I wouldn't mind if the functionality were in melt, but sometimes convenience functions are nice. Especially if you haven't used 'melt' in R, then you wouldn't look there. I certainly didn't. (I also find the melt docstring to be sparse, though the examples help immensely.)

C.f., and others http://stackoverflow.com/questions/17688155/complicated-for-me-reshaping-from-wide-to-long-in-pandas
 can you add a release notes entry reffing the issue? otherwise good to go
 Will try to remember when I get home tonight. Y'all need to hook something like this up. Life saver.

https://github.com/statsmodels/statsmodels/blob/master/tools/github_stats.py
 I tried this function to solve the stackoverflow question you linked to, but I get an error: http://nbviewer.ipython.org/github/jorisvandenbossche/scipy_notebooks/blob/master/pandas-pr-wide_to_long.ipynb#Test-example-of-stackoverflow (it's due to the fact that the second part of the column names (after the stub) are no ints but strings)
 Yes, this is a convenience function intended for proper panel (longitudinal) data where the stub variables are wide format timeseries data. I suppose there's nothing stopping this from taking a stub ending regex that defaults to ints though. I'll have a look.
 @jseabold merge this? hold off to look at handling ints? (or just raise as NotIMplemented for now)?
 Hold off, if you can. I should be able to get to it this weekend.
 ping @jseabold 
 Fixed the int conversion. Someone could likely make this more general. Also rebased.
 pls squash down and can merge

also if you want to put your docstring example in reshape.rst (if you think its needed), you can put a mention in v0.13.0 as well (up2u)
 thanks @jseabold 
