Type may now be either a String or a Number.

This is what the docstring for the Socket constructor claims, and is now correct.

Side Effect:
There is now a `_type` property on all returned Socket objects. `_type` is always a Number, corresponding directly to the ZMQ enum.

I could have opted to use the existing `type` property this way, but that seems of minor importance, and I decided compatibility with existing expectations was most important. If that should be changed, let me know.