[Refactoring] Use AS dasherize instead of gsub
signing with purpose via :for option

override == in SignedGlobalID to consider purpose

change serializer to JSON and clear the tests
Merge pull request #20 from tony612/signing-with-purpose

signing with purpose via :for option
Merge pull request #28 from seuros/master

[Refactoring] Use AS dasherize instead of gsub