ENH: Experimental CustomBusinessDay DateOffset class. Fixes GH2301.
ENH: Added check for required version of Numpy.
ENH: Added holidays and weekmask arguments to cdate_range as suggested by
@jreback. Also improved docstrings.
TST: Copied existing tests for BusinessDay for CustomBusinessDay to test that
the CustomBusinessDay class can be used as a drop-in replacement of the
BusinessDay class.

TST: Added tests for the holidays and weekmask functionality of the
CustomBusinessDay class.

TST: Added checks to skip tests involving CustomBusinessDay when this class
isn't available due to numpy requirements not being met.

TST: Added tests for holidays and weekmask functionality in cdate_range.
DOC: Added notes on the CustomBusinessDay class to the release notes as well as
the TimeSeries documentation.

DOC: Added more cautionary warnings about the experimental nature of the
current state of the CustomBusinessDay class.
DOC: release notes