Added fixes for SQL->DF, doc string, Py2 check
Added fixes for SQL->DF, doc string, Py2 check

Had to add several fixes for SQL->DF conversion, including setting the default
NaN type for floats to be float('nan') regardless of the users request.

Checked this with sqlite and postgres on both Py3 and Py2. Works fine.

rnelsonchem/simpledbf