Merge pull request #4 from jmaddi/master

allow string userid in User.authenticate
Swap ruby-hmac for openssl (ruby 2.0 compat)

Ruby-hmac has been deprecated [1]

[1] https://github.com/topfunky/ruby-hmac
Merge pull request #7 from invernizzi/master

Swap ruby-hmac for openssl (ruby 2.0 compat)
Update README for new version