feat(grunt-conventional-changelog): introduces grunt-conventional-change...

...log plugin

this plugin let us auto generate changelogs for this project.
all you have to do is to provide proper commit messages that
follow a given convention (like this one).

the angular project itself uses conventional changelogs to
automate this process.
to generate a changelog simply run `grunt changelog:fromRef:toRef` and
specify a release tag from  which the changelog
should be generated.

Example output: https://github.com/btford/grunt-conventional-changelog/blob/master/CHANGELOG.md
Conventions: https://docs.google.com/document/d/1QrDFcIiPjSLDn3EL15IJygNPiHORgU1_OOAqWjiDU5Y/edit