minor alias for grunt test
Memory leak fix in sigma.classes.graph

The edges referenced in the edges indexes where actually not the ones stored in the sigma.classes.graph instance, but the ones given to it.
Leak found by @sheymann and showed in #340 (thanks!).
Added a draft for CONTRIBUTING.md
Merge branch 'master' of https://github.com/Linkurious/sigma.js
Add plugins.helpers.graph
Merge remote-tracking branch 'origin/helpers.graph' into lk-sigma
Add helpers.graph build