added neo4j adapter

Hi all, 
after signaling an error on the wiki, I decided to add a new adapter.
Here you can find the code, BUT there is still the unresolved error I signaled in the Issue:

*    $ rails s  
    pdipietro@CUC-N4J-V4-0-0:~/gsn$ rails s
    simple_token_authentication/adapters/active_record_adapter
    simple_token_authentication/adapters/mongoid_adapter
    simple_token_authentication/adapters/neo4j_adapter
    bin/rails:6: warning: already initialized constant APP_PATH
    /home/pdipietro/gsn/bin/rails:6: warning: previous definition of APP_PATH was here
    Usage: rails COMMAND [ARGS]

Please, look at the error and then decide if you can merge this fork