EventSignatureViewSet

What is wrong?

Users should be able to make GET requests for individual retrieval and for list retrieval. class EventSignatureViewSet() is needed, so that it could be routed to '/event-signatures/' endpoint

How can it be fixed

implement class EventSignatureViewSet() similar to

ethereum-function-signature-registry/func_sig_registry/registry/api_views.py

Line 22 in f9491e6

class SignatureViewSet(mixins.CreateModelMixin,
write tests
route it to 'event-signatures' endpoint in https://github.com/pipermerriam/ethereum-function-signature-registry/blob/master/func_sig_registry/api_urls.py