archive strategy and changes to strategy

This adds the ability to specify a `--archive <url>` of a war/zip file to download and explode.

- Adds Archive strategy
- Writes `shasum` output to REVISION
- Some changes to how deploy figures out what strategy to use with a fallback of whatever strategy is specified (default of `git`)

This depends on changes engineyard/engineyard-serverside-adapter#4 for attributes on the adapter.