chore(grunt): adds grunt-ngdocs plugin

also integrates it into existing grunt configuration.
this is just the basic stuff, developer guide and tutorial
to come.
docs(ui.router.compat): adds docs for ui.router.compat module
docs(routerProvider): adds docs for routerProvider
docs(urlRouter): adds docs for urlRouter
docs(): wip docs for ui.router.state
Merge pull request #668 from PascalPrecht/ngdocs

ngdocs API documentation