Refactor/threshold

Changes:
- Threshold View.
- Loss & Forest2000 threshold.
- Forest2000 using new url.
- Widget Class improved with more features.
- All widgets using new Widget Class.
- LegendView renders now  in the correct order. Also it has been simplified a lot.
- Some url changes.
- Added some more methods to LayerSpecModel.
- Maptypes are now on a separated folder.
- Changed the way layers are rendered.

Added some comments inline.
Some css still need to be done for threshold. Merging now as this branch is getting too big.