Try to connect in Begin() too. Postpone error handling to Interceptor.

This change will improve Mongo and Revel plays nicer together in case Mongo is not started yet or restarted while Revel is running.

Dial() error is postponed until the Before() interceptor and Revel won't panic any longer.
Switched to config.stringdefault and warn when connection fails upon startup
Merge pull request #3 from hura/patch-1

Try to connect in Begin() too. Postpone error handling to Interceptor.