Add function signature to docstrings

A minor annoyance I have with `cytoolz` is that function definitions with function signatures are not available.  These are useful in a REPL, especially with `?` in IPython.  This PR adds signatures at the beginning of docstrings.  Signatures that are not automatically created are tested against `toolz` to verify they are up to date.

I merged this with #10 (bf388a2) to verify they are compatible.  They are.

`embedsignature=True` compiler directive is used to automatically add signatures to functions defined in pyx files.  Signatures were added manually to `cdef class` extension types, and `test_embedded_sigs.py` test was added to verify these signatures are up to date with `toolz`.