Simplify less, cleanup builds and implement r.js in Scala

This started as cleanup of bokeh's less, because previously we used imports and had some paths hard-coded in builds. Now we use only imports. Then I did minor cleanup of `Gruntfile` and removed unneeded stuff from sbt build. And then, being annoyed by r.js run times, I implemented (a subset) of r.js in Scala. To use it, issue `./sbt requirejs`. Tests pass using `bokeh.js` generated this way. Currently the biggest issue is resolving cyclic dependencies. Currently any kind of cyclic dependencies are allowed. A warning is issued when this happens (and yes, bokeh's dependency graph contains cycles, mostly due to `common/base`), but the resulting dependency order potentially may be wrong. Use with caution, at least for now.