Adds FitsTypeMatcher

This matcher is to compare two instances by type.
Prevent panic on comparing nil values
Rename FitTypeOf to BeAssignableTo
Adds godoc documentation to BeAssignableTo
Cleanup unused types and verbose comments
Update specs for AssignableTo matcher
Update error messages to reflect semantics
Rename BeAssignableTo to BeAssignableToTypeOf
Adds test and doc for custom type
Merge pull request #4 from pjvds/master

Adds AssignableTo matcher