Add '--force' option to download/parse links found in a remote site

Force mlstats to download and parse any link found in a remote site
(not valid for Gmane links nor local files).
Add support for Apache's 'mod_mbox' plugin interface

Links from Apache's 'mod_mbox' plugin contain trailing
"/thread" substrings. Remove them to get the links where
mbox files are stored.
Fix error retrieving archives from the last month in Apache's mod_mbox
Analyze only non-visited mbox archives
database: add mailing_list_url to primary key in messages table

This change on the database schema allows to store messages
that were sent to different mailing lists.
Typo fix and removed some trailing white spaces
Merge pull request #25 from justinclift/master

Typo fix and removed some trailing white spaces
htmlparser: Accept gzip encondings. Fixes #11