Merge pull request #22 from reneschuster/master

Plugins: ClamAV, Postscreen and SpamAssassin small changes
Merge pull request #23 from reneschuster/master

Plugin SpamAssassin: Small changes and README update
Merge pull request #24 from reneschuster/master

Plugin OpenDKIM: Updated license header for new year and README update
Plugin SpamAssassin: Fixed dbMigration, changed version_compare to 0.2.5
Plugin RoundcubePlugins: Version check for Roundcube. Plugin now compatible with Dovecot 1 and 2. Small fixes. Updated license header for new year and the README file.