Adding Firebase (instead of Pusher) as data synchronization and storage ...

...service

* New firebase functions create new firebase connection, set data, and get data
* Converting Sequence constructor to Song, as each song will now have four sequences (one for each instrument/user)
* Adding sequences hash for the four sequences, one for each channel/instrument/user
* Adding firebase js via Firebase CDN to application.html.erb
* Changing note off values in array from null to -1 as data sent to firebase is converted to a string