Added BIF.py into readwrite
Added BIF.py into readwrite

All functions are not implemented. only get_variable,get_states and
get_property are implemented.

anaviltripathi/pgmpy,sandeepkrjha/pgmpy,liquidmetal/pgmpy,jhonatanoliveira/pgmpy,jhonatanoliveira/pgmpy,ankurankan/pgmpy,kris-singh/pgmpy,pgmpy/pgmpy,pgmpy/pgmpy,liquidmetal/pgmpy,anaviltripathi/pgmpy,khalibartan/pgmpy,ankurankan/pgmpy,kris-singh/pgmpy,sandeepkrjha/pgmpy,abinashpanda/pgmpy,abinashpanda/pgmpy,khalibartan/pgmpy