Adding explicit declarations of bundle gems

Addressing: https://github.com/projecthydra/hydra/issues/5
Renaming hydra.rb -> hydra6.rb

Prior to this change, the `require 'hydra'` declaration did not load the
hydra gem's lib/hydra.rb but instead loaded hydra-head/lib/hydra.rb.
Adding finalized copyright statement
Updating to most current contribution guidelines
Adding Acknowledgments and Hydra byline to README