parsing weeks

This PR does two things:

1. Adds support for `get()` and `set()` for a bunch more properties
2. Adds parsing support for 'gg', 'GG', 'w', 'W', 'e', 'E',  and 'd'. See #850 and #689

There's a bit of undefined behavior here, like what happens when you have contradictory strings, per @timrwood's comment [here](https://github.com/moment/moment/issues/850#issuecomment-19649965). I think that's probably OK.