This hook makes sure we add a value to the addthis field when there is no
value. We need this value to make sure we are able to render something.

The markup is now create through a seperate hook function. This is more
dynamic and creates the possiblity to add the theme function better.

Todo: we need to test settings for a formatter and add a field instance to
the block. Settings need to be generic and also be able to be set on the
theme function.

Committing contributed code by Matthias Glastra: Mostly coding conventions related modifications and renaming to prevent possible name collisions.

- Using field formatters to select the sharing widget type in the UI (contributed by Matthias Glastra, matglas86, http://drupal.org/user/573464)

- Decided not to use instance level settings as having one type of sharing widget per entity instance type is more logical in most cases
  - There might be some special cases where you actually need to have a different type of widget for entities of the same type, but currently we try to keep this as simple as possible

Removed hook_field_instance_settings_form, as we do not want to add anything extra to the instance settings form.

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