follow up from review #10 #12

#10

* Added drupal_static for comment_sources_get_source_node()
* Added module dependencies
* Fixed form_alter submit callback assignment
* Documented & renamed $trim_preset parameter in API functions
* Simplified form_alter display logic (removed unnecessary if/else for
identical scenarios)
* Used named constant, not integer, where applicable
* Removed unused function _comment_sources_node_form_submit()
* Removed unused vsite parameter in comment_sources_set_type_source()