moving `assets` into `src` directory

the `src` directory will be shared between plugins. it includes pre-compiled and compiled sources and now assets.
changed name of LESS file to better reflect purpose
plugins will expose compiled resources via `dist` to other plugins

solutions to plugin references:

- CoffeeScript - no access
- LESS - access at compile time to `../../plugins/`
- JavaScript - managed by `build.coffee` and `requireJS`
- CSS - can be used from `plugins/` directory from HTML
Merge branch 'features/pluginable'