add missing type assertions on globals
support BigFloat and BigInt conversions (fixes #13)
allow custom pytype return values

example usage (for SymPy):
type Sym
     x::PyCall.PyObject
end
Sym(s::Sym) = s
convert(::Type{Sym}, o::PyCall.PyObject) = Sym(o)
PyCall.pytype_query_add(sympy.basic[Basic], Sym)
in typesymbol, visit the module hierarchy without reversing the order
make pytype_query_add more dictionary like and rename it pytype_mapping