Tslibs resolution
One more step towards getting `tseries.frequencies` into cython (and getting rid of non-cython dependencies of tslib/period).  Also moves towards de-coupling period from tslib.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
