Add command: `octopress deploy`

Give Octopress some command-line-based deployment :heart:!

Relevant issue on octopress/octopress: https://github.com/octopress/octopress/issues/4

- [x] Implement
- [ ] Implement piping on `octopress`: https://github.com/octopress/octopress/pull/5
- [x] Fix #1.

Add `octopress-deploy` to your `Gemfile` in the `:octopress` group and it's accessible!