Support more than one deploy method at a time.

Hey @opsb,

I've created this pull request to collect your strategies feature into one place. Please submit additional pull requests targeted at the strategies branch. They should automatically show up here as soon as I merge them. We'll use this pull request to discuss the big picture. If you have a better idea about how to coordinate this I'm all for it.

As for my previous comment about how to handle backwards compatibility in the deploy blocks in config.rb https://github.com/tvaughan/middleman-deploy/pull/42#issuecomment-26872857, I have an idea. One way to solve this problem is to keep the current `activate :deploy` block, and add a second `activate :deploy_with_strategies` block. Food for thought.

Thanks!