[WIP] Be2bill onsite support

Todo:
- [ ] Check HASH: it's weirdly not working at my place, I got a "Bad HASH" response from be2bill but I don't see why. Does anyone have a be2bill testing platform and is willing to test it?
- [ ] Implement a switch between server-to-server and on-site form methods: I don't know how to do it properly, for now I just override the previous be2bill method but the aim is to have both running in parallel (developer's choice)
- [ ] Handle to feedback from be2bill? cf next

About be2bill "feedback":
* When the user submits the form on be2bill side, he comes back to our site with all needed parameters in the URL (if payment was OK, etc.)
* At the same time, be2bill sends us an HTTP request with these same parameters. This one is more reliable of course (because it comes from be2bill itself)
=> Question: which one do we listen?