Version 1.6, alpha, revision 2

Removed instances of inserting into file_tag with NULL tags (Closes #33)

• This change reduces database file sizes by as much as half by not
  storing the many duplicate rows that resulted from having a NULL-able
  primary key in the file_tag table.
• Added data and schema migration for removal of NULL tags. Incremented
  DB_VERSION.
• Moved files_by_id and loading from the sqlite database back into tagdb
• Stopped creation of log files from test_tagdb