Add exclusion in tag search

  * Searching '-mytag' will now exlude all shaares with 'mytag' tag.
  * All tags starting with a '-' are renamed without it (through the Updater).
  * Unit tests.

Minor code changes:

 * LinkDB->filter() can now take no parameters (get all link depending on logged status).
 * tagsStrToArray() is now static and filters blank tags.