First attempt to integrate nvd3.
Merge branch 'master' into nvd3
New attempt using Rickshaw.
Personal replacement.
Almost done.

Few issues remain:
* How to resize the grid according to brush selection?
* The tooltip position is not correct when a selection is active
* Curves sometimes overflow
New d3 charts are already done.
Remove last rickshaw references.
Merge branch 'master' into d3js
Little translation.
Documentation (not complete).
Merge branch 'master' into d3js

Conflicts:
	modoboa/extensions/stats/grapher.py
Wrong translation fixed.
Merge branch '1.1.x'
Merge branch 'master' into d3js
Cleanup.

see #492