- Added hook to add additional URL abbreviation services.
 - Added tr.im URL abbreviation service.
 - Minor related cleanup.

- Documentation improvements.

  - Admin can now fully customize the Tweet text
  - API change

- retrieved URLs are now cached
- conditional code loading in 6.x
- 5 new URL abbreviation services added
- new option to choose primary and backup URL abbreviation service
- ability to exclude certain nodes from showing the Tweet link