Implementation of plugin chaining.

- Process modifiers now passes the activity and a message that can be modified as modifiers are applied to.
Fixes MySql support

- Removed index on migration because the index is removed on the later migration anyways and MySQL doesn't like indexes on text columns.
- Fixes #207
Merge pull request #221 from gabceb/kandan-219

Implementation of plugin chaining.
Merge pull request #222 from gabceb/kandan-207

Fixes MySql support