Merge tag 'v0.1.30' into develop

Improved Slug plugin. It suppors multiple inputs
Rebuild slug if it changes
Improved slug update
Minor change in the test
Added @First and @Last for Plugins
Improving internal code
Adding @Last to Sluggable plugin
No I/O is required to calculate the namespace

Avoid IO/sha1 call to get the namespace *when* mapper is already generated (default case)
Update namespace info
Merge branch 'master' of github.com:crodas/ActiveMongo2 into develop
bump version
Merge branch 'release/v0.1.31'