Reduce gem dependencies

[ipoerner]

Currently, ActiveMailbox depends on the following gems:

• ActiveSupport
• RMail
• TMail
• (Rake)

Some additional Rake tasks also depend on

• RDoc
• Rcov

That's a whole lot of dependencies, and the list can probably be reduced, i.e. by offering ActiveSupport integration on availability (i.e, when specifying configuration values like 5.seconds) and go with a best effort approach otherwise.

Also, either one of RMail/TMail should be optional (however, I think RMail actually was optional, need to look this up).

Rcov is pretty old and used not to be Ruby 1.9 ready when I last looked at it, so it may need to be dropped at some point, although it's an awesome tool.