update to holiday to help with GH#7070
I am working on a financial calendar and wanted to add the following functions to pandas.tseries.holiday in order to properly handle half days.

looks reasonable (test failures though)
 The right syntax would be `h = Holiday("Easter Monday", month=1, day=1, offset=[Easter(), Day(1)])`.  However, I'm seeing a problem in python3 with the map objects.  I think if we just wrap list() around the map in Holiday as it iterates through the offsets everything will work.
 Thank you very much. Should I keep a test for defined holidays?
 I don't see any harm in that.
 Thanks for the help on this.
 @rockg, I am still crashing on a python3 build.  How did you intend to handle the two observations?
 I will setup a python3 environment to test both going forward.
 Yeah, I had to do the same.  So many of my changes in the past needed
python3 specific fixes.
 @MichaelWS looks fine, can you add a release note (just to document)
 easy enough, should I put it in v0.15.0.txt?
 yep
 all green, I added to the init so its clear

@jreback , thanks for the help.
 oh i see, ok then.
 thanks for the PR!
