Install support for X-Failure call-back URL.

To be called when par2 verification fails, the RAR files have CRC errors or the right password isn't known.
Set new_nzb_on_failure option default off.
Implement support for X-Failure call-back URL.

Optionally to be called when par2 verification fails, the RAR files have CRC errors or the right password isn't known.
Update text files for 0.7.18RC1
Additional logging for folder renaming.
Update main POT file.
Merge remote-tracking branch 'upstream/0.7.x' into 0.7.x