Begin to decouple plugins from the Annotator namespace

Part of the complexity removed in the previous commit was the result of
needing the ability to build individual plugin files which added
themselves to the Annotator namespace.

This commit starts to unpick some of this complexity, by allowing
plugins to register themselves in a singleton "plugin pool" by name,
rather than somewhat hackily adding themselves to a global shared
object. The result is broadly similar, although this feels cleaner and
more explicit.

One implication is that plugins *must* now be loaded after Annotator
core, although I don't see that this will cause many problems.

As of this commit, inter-plugin interactions are still broken.
Remove unused file: console.coffee

This is no longer used by anything.
Standardise on require('foo') over require 'foo'

Sorry, this was getting on my nerves...