add use_bultin_types option

[anthrotype]

Fixes #162

This is True on python3, and False on python2 by default.
Can be set explicitly to obtain the same treatment for bytes vs data on both py2 and 3.

[anthrotype]

and of course this also adds the plistlib.Data class, that wraps bytes and allows to distinguish them from str on python 2 (or when use_builtin_types is False).

[anthrotype]

i'll tag a new release tomorrow