- Added tests
- Added some check_plain()-calls

- Refactoring and renaming

- Renamed AddThis username to profile ID, as this is more consistent with the current API described at addthis.com
- Moved some literals to AddThis-class as constants to reduce literal duplication

- Moved AddThis username configuration under Configuration / System / AddThis (previously under field settings)
- Changed widget type selection to use 'select' instead of 'radios'
- Added tests