CLN: Refactor pd.offsets.CustomBusinessDay

[bjonen]

xref #8293

The code has evolved quite a bit and could probably benefit from refactoring.

In particular warpping the busdaycalendar logic in a wrapper class could make the code easier.

Notes about the current implementation:

• CustomBusinessDay uses np.busdaycalendar to do the computations.
• To speed up creation of new CustomBusinessDay offsets the calendar is cached in kwds.
• To check equality of two CustomBusinessDay instances, the attributes holidays and weekdays are used.
• remove EXPERIMENTAL (in docs, but check off in the experimental issue EXPM: re-evaluate experimental #5049)
• might be some docs that say requires numpy >= 1.7 (but now all of pandas on that, so can cleanup)

[bjonen]

From the previous discussion
Quoting @jreback

hmm sounds like we really should have is a wrapper object 
around np.busycalensar that is more friendly 
(and does these types of validations) and can cache itself too -
this is pretty straightforward btw

just a

class Calendar(object):

holds the actual np.busycalendar
and weekmask/holidays

[jreback]

@bjonen if you want to do a PR for the last point up their (any numpy 1.7 references in docs ok by me)