Ad issue #24: Collect messages from TikZ externalisation now.

Along the way, noticed and fixed several issues:

 * TeXLogParser is now thread-safe.
 * TeXLogParser can now restrict its output to messages
   from lines between two lines matching specified regexps.
 * TeXLogParser now gives highest precedence to messages in
   file:line format.
 * TeXLogParser did not catch all opening files since it only
   matched one opening file per line. Remedied.