Improved styling and functionality for the library and renamed a few classes for clarity

The button to add a file now matches the design.
Added an extra sass file for the library, for details that the panels don't share.
Renamed 'views/ui/library' to 'views/panels/mediaLibraryPanel', because there were multiple library.js files. See 'issues' on Github. https://github.com/projecttouch/projecttouch/issues?state=open
Renamed 'views/ui/media.js' to 'views/panels/mediaLibraryItem' for similar reasons, and a few more.
Added proof-of-concept playback functionality to mediaLibraryItems. In the future we should have a central collection of video items to playback on canvas via requestAnimationFrame.
Rendering mediaLibraryItems via Underscore template.