Raise subject field from 255 to 320 chars

I parsed the developer mailing list of the TYPO3 project via
`mlstats --report-file=report.txt --db-user root --db-password "" --db-name mlstats http://lists.typo3.org/pipermail/typo3-dev/`

and got the message
```
...
Analyzing /home/vagrant/.mlstats/compressed/lists.typo3.org/pipermail/typo3-dev/2004-July.txt.gz
Analyzing /home/vagrant/.mlstats/compressed/lists.typo3.org/pipermail/typo3-dev/2004-August.txt.gz
Analyzing /home/vagrant/.mlstats/compressed/lists.typo3.org/pipermail/typo3-dev/2004-September.txt.gz
/usr/local/lib/python2.7/dist-packages/mlstats-0.4-py2.7.egg/pymlstats/db/mysql.py:204: Warning: Data truncated for column 'subject' at row 1
Analyzing /home/vagrant/.mlstats/compressed/lists.typo3.org/pipermail/typo3-dev/2004-October.txt.gz
Analyzing /home/vagrant/.mlstats/compressed/lists.typo3.org/pipermail/typo3-dev/2004-November.txt.gz
Analyzing /home/vagrant/.mlstats/compressed/lists.typo3.org/pipermail/typo3-dev/2004-December.txt.gz
...
```

After some debugging i saw that the subjects got a length from 292 up to 299 chars.

This PR extends the subject field of MySQL to 320 chars.
In data storage terms this only means one additional byte at the end of the field. See http://dev.mysql.com/doc/refman/5.1/en/storage-requirements.html