BUG/ENH: allow axis name aliasing

[jreback]

http://stackoverflow.com/questions/15533093/pandas-access-axis-by-user-defined-name/15534409#15534409

not that this is user friendly, but

pd.DataFrame._AXIS_ALIASES['myaxis'1] = 0
pd.DataFrame._AXIS_ALIASES['myaxis2'] = 1

df.sum(axis='myaxis1')

should work but doesn't

[jreback]

PR #3110 fixes this