ported backward compatibility classes and properties from php-mf2

Filled in the backward_compat module with class mappings adappted
from php-mf2. BeautifulSoup tags are augmented with mf2-style
classes before the usual parsing takes place.
Change backcompat new-properties to lists
Allow BeautifulSoup document, in addition to a file handle or
string, in the mf2py.Parser constructor
Merge branch 'master' of github.com:kylewm/mf2py into backcompat