Setup bower for polymer

* Created bower.json file with the dependencies currently in use in testrtc/index.html.
* Added bower_components to .gitignore.
* Added ignore rules to the lint plugins for the bower_components folder.
* Changed links in index.html to point to locally downloaded polymer files.
* platform.js has been renamed to [webcomponent.js](https://blog.polymer-project.org/announcements/2014/10/16/platform-becomes-webcomponents/), updated it accordingly.

How to:
* Install [bower](http://bower.io/#install-bower) (requires [node.njs](http://nodejs.org/))
* Navigate to the ../testrtc/ folder in a console/terminal
* Type "update bower"

All the dependencies in bower.json will be downloaded to ../testrtc/bower_components. Now it's ready to be used locally.