Implement #30

This branch implements #30, turning the parsed parameter into an object, Method::Signatures::Parameter.

This isn't ready to be merged, MS::Parameter needs unit testing.  Just putting it in here to begin the review process and see if anyone else wants to work on it.

It also slows down Method::Signatures load time and signature parsing by about 10%.  I'm not terribly worried about that.  This was a rote refactoring and that's just what you lose switching from hash lookups to method calls.  Once the parameter parsing makes more sense I'm sure we can optimize it back and then some.