Changed the map_field decorator to assume the method name is the from_field if None is provided.
Updated Resource.convert_to and Mapping.convert to allow field values to be passed in.

This allows fields values to be specified externally from the mapping inline (making it easier to do conversions in a list comprehension)
Merge branch 'master' of github.com:timsavage/odin
Bumped version for a future release