Changed message_body from TEXT to MEDIUMTEXT, because we got messages > 65.535 bytes

Hey,

during analysing the TYPO3-Dev mailing list (http://lists.typo3.org/pipermail/typo3-dev/) i got some output like:
```
...
Analyzing /home/vagrant/.mlstats/compressed/lists.typo3.org/pipermail/typo3-dev/2013-January.txt.gz
Analyzing /home/vagrant/.mlstats/compressed/lists.typo3.org/pipermail/typo3-dev/2013-February.txt.gz
Analyzing /home/vagrant/.mlstats/compressed/lists.typo3.org/pipermail/typo3-dev/2013-March.txt.gz
Analyzing /home/vagrant/.mlstats/compressed/lists.typo3.org/pipermail/typo3-dev/2013-April.txt.gz
/vagrant/MailingListStats.git/pymlstats/db/mysql.py:210: Warning: Data truncated for column 'message_body' at row 1
  self.write_cursor.execute(query_message, values)
Analyzing /home/vagrant/.mlstats/compressed/lists.typo3.org/pipermail/typo3-dev/2013-May.txt.gz
Analyzing /home/vagrant/.mlstats/compressed/lists.typo3.org/pipermail/typo3-dev/2013-June.txt.gz
...
```

For the complete analyse i got 4 of this messages.
After some debugging i found that the mysql field messages.message_body is TEXT and can store 65.535 bytes bytes.
In the TYPO3 dev mailing list some messages are longer like:
* [[TYPO3-dev]  Import Selection with TYPO3 Element Browser in 6.04](http://lists.typo3.org/pipermail/typo3-dev/2013-April/046711.html): 72.029 chars
* [[TYPO3-dev] Tester needed : gridelements_fce + generator](http://lists.typo3.org/pipermail/typo3-dev/2014-April/048265.html): 788.257 chars

Of course, the content represents some crypted stuff, but this does not matter.
In my opinion is the content important, because the meaningful of the content depends on the question.