Allows simple not rendering via input box

The present commits allow the user to input notes to be rendered into the last measure available.
The documentView shows a new input box in which to enter notes with the syntax: 

pitch num/den,pitch num/den, ...
eg:
69 1/4,70 1/2,71 1/4

When the measure is complete, the user can press enter to render the new measure into the track

The present commits also correctly save the track data into a JSON file 

In development:
- Dont save the whole $scope data structure, only needed properties
- Improve the user interface (text box)
- Fix the track overflow bugg