Implement UpdateDocument and AbstractDocument

* VuFindSearch/Backend/Solr/Document/DeleteDocument.php: New
  class. SOLR delete document.
* VuFindSearch/Backend/Solr/Document/UpdateDocument.php: Extend
  abstract base class.
* VuFindSearch/Backend/Solr/Document/AbstractDocument.php: New
  class. Abstract base class of SOLR documents.
Implement generic write() operation

* VuFindSearch/Backend/Solr/Connector.php (update, save): Removed,
  superseded by more generic write().
  (write): New function. Send document to update handler.
Remove Connector::delete()

* VuFindSearch/Backend/Solr/Connector.php (delete):
  Removed. Superseded by more generic write().
Remove Backend::delete()

* VuFindSearch/Backend/Solr/Backend.php (delete): Removed.
Merge branch 'search-refactor' of ssh://vufind.git.sourceforge.net/gitroot/vufind/vufind into search-refactor
Dependency injection for related record modules.