Merge pull request #3176 from jreback/resample_dtype

TST: resample test custom_grouper haveing dtype comp error on 32-bit
BUG: fixed issues where passing an axis of 'index' would fail (as its expecting an axis number)
ENH: added ability to use the index name as the name of the axis
     added set_axis_alias/clear_axis_alias to set for an object type (e.g. DataFrame/Panel),
       aliases at run-time
     GH 3109