Initial implementation of :main functionality, fix #14

- Fix bug where tag src attribute not relative to main JS file (#14).

Main entry point implementation:

- Create a file called app.main.edn in your source paths, with the
  following contents:

    {:init-fns [app/doit]}

- Then make a ClojureScript namespace, app.cljs with a doit function.

- Add a script tag like <script src="app.js"></script> to your HTML
  file. (This is the only script tag you need.)

- When you compile with the cljs task it will create an app.js file that
  will require the app CLJS namespace and call the doit function with no
  args.