License updates

- Added a LICENSE.txt
  (See "How to apply this license" at
   http://choosealicense.com/licenses/gpl-2.0/)

- Added license headers to files where missing.

- Used /* */ comments for license headers. (Allows folding in
  QtCreator.)

- Replaced BSD template from `harbour-ttrss.qml` with GPL.
Merge pull request #88 from michael-k/license

License updates