Multi extruder support for GCODE viewer

Filament usage calculation per layer is still buggy though
Refining temperature control REST API, also added support for multi extrusion while at it
Added pusher.color.js
Lots of changes:

- GCODE viewer now supports multiple extruders and configurable bed sizes
- GCODE viewer also now based on callbacks and config injection instead of dependence on elements in the embedding website
- Switched GCODE viewer to bootstrap-slider for the layer/command sliders, patched bootstrap-slider a bit up (TODO: PR to upstream)
- GCODE interpreter in backend also calculates filament usage for all extruders
- More consequent use of LESS, switched bootstrap to LESS source to have access to variables and mixins, introduced config setting to switch between usage of compiled CSS (checked in as well) or LESS, defaults to CSS