Adds known-projects system

Known projects makes it easy to return to projects that you have
previously done work on. Projects are saved whenever the system
already does caching. The main user-facing component is the
`projectile-switch-project` command, bound to 's' on the projectile
mode map, which makes it easy to switch to the new project.
Merge pull request #69 from joelmccracken/add-start-and-stop-commands

Project Bookmarks, and Project Starting and Stopping