Convert Resolved Context Status to Enum

I have converted the `Resolver` and `Solver` status attributes to use an enum (`ResolverStatus` and `SolverStatus` respectively) instead of a string.

This requires the use of `rez.vendor.enum` which is an [implementation of a new python 3 enum feature](https://pypi.python.org/pypi/enum34).

Based on the need for POD, I have updated the `to_dict` and `from_dict` method in the `ResolvedContext` class.  As best I could tell - this is the only entry point for serialisation.

The tests pass.