Add the build status in the README

The file README.md has been updated with the travis-ci status.

Bug-Url: https://github.com/dartdesigner/Dart-Designer/issues/10
Change-Id: I49f552e82e5915a8a2e8d23d65f004079f940e6b
Add tracking code

The code to track the usage of the Dart Editor on Google Analytics has
been added to the application.

Bug-Url: https://github.com/dartdesigner/Dart-Designer/issues/11
Change-Id: Ie235bc79fdc08497b5a66de8491512164d540410
Add help pages

The help pages are now located on
https://github.com/dartdesigner/documentation

Bug-Url: https://github.com/dartdesigner/Dart-Designer/issues/6
Change-Id: I0d7e6e502074aa3047e5f4d8de8808befc73eb2c