Add support for travis-ci build

The travis.yml file has been contributed.

Bug-Url: https://github.com/dartdesigner/Dart-Designer/issues/8
Change-Id: Idd09771cd8b94aaa66c81bd2b4eda0ad0352670e
Improve the contributing guide to describe commit messages

The file contributing.md has been updated.

Bug-Url: https://github.com/dartdesigner/Dart-Designer/issues/9
Change-Id: I16dce2bec3b4b1963ce46b99562caef2884ed239