Display main libs versions in debug log

We were already displaying pyqt, mutagen and discid versions in About dialog, now they'll be shown in debug log too.
It will help with debug logs provided by users.

**Before** this patch, debug log output starts with:
```
D: 10:38:54 Debug mode on
D: 10:38:54 Starting Picard 1.3.0dev4 from '/home/zas/src/picard_zas/picard/tagger.py'
D: 10:38:54 Platform: Linux-3.13.0-24-generic-x86_64-with-LinuxMint-17-qiana CPython 2.7.6
D: 10:38:54 Configuration file path: /home/zas/.config/MusicBrainz/Picard.conf
```

**After** this patch, debug log output starts with:
```
D: 10:40:04 Debug mode on
D: 10:40:04 Starting Picard from '/home/zas/src/picard_zas/picard/tagger.py'
D: 10:40:04 Platform: Linux-3.13.0-24-generic-x86_64-with-LinuxMint-17-qiana CPython 2.7.6
D: 10:40:04 Versions: Picard 1.3.0dev4, PyQt 4.10.4, Mutagen 1.22, Discid discid 1.1.0 (compat layer from python-libdiscid 0.4.1), libdiscid 0.6.1
D: 10:40:04 Configuration file path: /home/zas/.config/MusicBrainz/Picard.conf
```