Place plugin classes in RedmineTrello module

To avoid clashes with other plugins, all plugin
classes have been namespaced. In the previous setup,
a Redmine or plugin class called "Hooks" might've broken
our entire plugins functionality, which is now no longer
the case.
Create a formatter class to generate trello card
Don't catch exceptions on Trello card creation

This seems to be preferable since, with the previous
solution, it wasn't immediately obvious when Trello
card creation failed. As soon as somebody notices one
could look at the logfiles, but by then dozens of issues
might've already been created.
Merge pull request #2 from x3ro/improved-card-creation

Improved card creation and more!