Fixes pypa#659

Let me know if something like this seems acceptable to you and I'll add
tests and documentation.

Some notes:
- My use case was adding a local version to an existing version
- I don't think it would make sense to allow replacing parts of the
  release, e.g. "major", since I don't really see a use case. Things
  like `v.replace(major=v.major+1)` are probably just mistakes.
- This is why I don't allow replacing `epoch` either
- `Version.__new__(Version)` is a little gross, as is the munging.
  The munging is designed to line up with the corresponding properties,
  so `v.replace(xyz=v.xyz)` always works.