WIP,CLN: com.ReprMixin everywhere, cls defines __unicode__ and py2/3

[ghost]

str,bytes,repr work.

reduces code duplication and boilerplate.
still need to replace

class Foo(com.ReprMixin,object)

by

class Foo(PrintableObject)

lots of churn, it'll wait till 0.12.

[jreback]

@y-p in 0.12 can do this before my #2915 where I moved all these routines to generic (as part if PandasObject)

but I'll just rebase after u merge and take your changes

[ghost]

I think we'll do a "big change" merge period right after 0.12, some things have
accumelated.

This is trivial to fixup compared to your refactor. after you...