Add License.txt for MIT license

Created by: kreynen

I see that this project is licensed at MIT in https://github.com/gitterHQ/desktop/blob/master/package.json#L30 and the bottom of https://github.com/gitterHQ/desktop/blob/master/README.md, but you aren't including a License.txt. Github now parses those files and displays the license in the UI.

Not having the License.txt with the copyright notice is a little odd since including the copyright notice when modifying the code is a requirement of the license.