[WIP] Refactored web server

It's time to refactor the internal web server. Main pull requests objectives:

### Pure-Python part
- [x] Replace CherryPy by Flask
- [x] Refactor the ```web``` package (moved onto ```cr.web```)
- [ ] Refactor the API

### Assets
- [ ] Dynamic generation of assets using Node.js and Grunt (CSS and base JS are built in the [CentralReport/cr-design project](https://github.com/CentralReport/cr-design/))
- [ ] Replace the old design by the new one ([CentralReport/cr-design project](https://github.com/CentralReport/cr-design/))
- [ ] Pre-built assets when generating packages

### General
- [ ] Update related documentation