Introduce the Figwheel client

[arichiardi]

Ok this is a super alpha of the figwheel client in boot-reload.

At the moment the implemented server to client messages are:

• [x] :files-changed
• [x] :compile-warning
• [x] :compile-failed
• [x] :css-files-changed

Whereas the implemented client to server messages are:

• [ ] "file-selected"
• [ ] "callback"

Other tasks to complete:

• [x] Inject the Figwheel bootstrap script
• [x] Handle individual js-onload per build id (untested but there)
• [x] Figwheel version
• [x] Use Figwheel init code (?)
• [ ] Handle boot-reload's :asset-host in Figwheel (link to comments)
• [ ] Pass the right :open-file option to Figwheel
• [ ] Solve the "first message lost" problem with a message queue (?)
• [x] Assert needed dependencies
• [ ] Repl integration (?)

To be tested:

• [ ] Node client
• [ ] Web-worker client
• [ ] Trigger of multiple js-onloads

Moreover, it requires boot-cljs for this patch.

It is here for @Deraen (and every other who wants of course) to provide early feedback and for very very brave people to give it a spin and report errors.