Add test that compares docstrings of functions in `toolz` and `cytoolz`.

This is to improve maintainability.  If docstrings don't match, then a
convenient diff is printed.  Docstrings in `cytoolz` may have additional
lines at the beginning and end.
Add function signature to docstrings.

`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`.
Merge branch 'docstring_test' into embedsignature
Simplified "test_docstrings.py" and "test_embedded_sigs.py".

The tests in those files shared similar, somewhat convoluted logic.  A more
functional approach is now done, which makes it more clear and more concise.
Test all cytoolz docstrings for signature at beginning followed by one blank line.

Also, don't use `dict.has_key`, because it's not in Python3.
Merge branch 'master' into embedsignature

Conflicts:
	cytoolz/dicttoolz.pyx
	cytoolz/itertoolz.pyx
Merge pull request #11 from eriknw/embedsignature

Add function signature to docstrings