Raster import at editor

This covers all issues related with importing rasters from CartoDB Editor:
https://github.com/CartoDB/cartodb/issues?q=label%3Araster-import+is%3Aclosed

It's missing more tests on the raster import part, but is stable enough (and manually tested) so we can at least try at staging.


Release steps:
1) Release extension 0.5.0
2) Release branch
3) Run as usual cartodb:db:load_functions rake
4) Run cartodb:db:grant_general_raster_permissions rake
5) activate raster import at app_config.yml:
```raster_import_active: false``` (see app_config.yml.sample)