Refactoring: Moved field hooks and block hooks to separate include files. Created a subfolder for all include files.

Removed some legacy hook implementations that were not doing anything useful. Let's implement these only when required.

Adding AddThis configuration script only once per page. Previously this was added once for each AddThis field on page.

- 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

Modified implementation of hook_link not to generate actual link and changed default HTML attributes to generate valid XHTML

Applied patch by Likeness to let users choose if Addths button is visible in teasers, changed advanced configuration fieldsets collapsed by default and some code cleanup