Use QThreadPool for threading
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
Merge pull request #146 from mwiencek/new-threadpool

Use QThreadPool for threading
_stderr_receiver(): improve reporting of UnicodeDecodeError
_message(): do not convert unicode strings

Receivers have to take care of conversion instead, if needed.
Move log and logview code to classes, to ease re-use.
show_log(): minor tidy up
Add a logger for status history
Add a window for status history display.

Launch from Help menu for now
Add a function to format a log line in human-readable form
Use log.formatted_log_line() in LogViewCommon
Use formatted_log_line() in _stderr_receiver()
Do not display level prefix for history (always LOG_INFO)
Remove a superfluous import of picard.util.queue

It has been removed, so this import made picard unstartable on clean
checkout (old ones most likely has a .pyc file of the queue module lying
around).