x86 - > x64 Payload injection

This modified payload_inject to allow x86 -> x64 injection when https://github.com/rapid7/meterpreter/pull/76 is landed.

Slight refactor to use the post mixin, and refactor the mixin to return the thread object so we can do things like WaitForSingleObject on the thread etc.

### Verification
- [x] Works when creating x86 threads from x86 Meterpreter.
- [x] Works when creating x86 threads from x64 Meterpreter.
- [x] Works when creating x64 threads from x86 Meterpreter.
- [x] Works when creating x64 threads from x64 Meterpreter. (On 2012!)