Don't throw an AttributeError if no discid module is available

The cd lookup button was enabled even if no discid module was available,
don't do that.

Additionally, display information about the missing discid module in the
About window.

This fixes PICARD-510
Regenerate picard.pot
_save_images: `continue` after deciding a file doesn't need to be saved

The previous code was `return`ing from the function, so images after the
first already existing one were not even considered for saving.
Add '/' to _escape_lucune_query in webservice.py

Due to Lucene 4 updates, this is necessary.
Merge pull request #148 from ianmcorvidae/patch-1

Add '/' to _escape_lucune_query in webservice.py
Use separate thread pool for file saving

With a thread count of 1, to avoid race conditions in File._save_and_rename.
Merge branch 'master' into new-threadpool

Conflicts:
	picard/ui/mainwindow.py