Review security providers API

Replace encode/decode by serialize/deserialize and add missing verify
method.
Add decode/encode methods to security providers API
Implement verify method for none security provider
Implement pymco.security.SSLProvier.verify
Add integration tests for SSL security provider
Fix SSL security provider integration tests

They were running with none provider instead.
Python 3 SHA objects need bytes instead of string
Freeze stomp.py requirement

Until current_host_and_port issue (#10) is fixed.
Merge pull request #5 from rafaduran/feature/review_security_api

Review security providers API