feature(grunt-release): Adds grunt release task

Had some time yesterday and drafted a grunt release task.

You can safely play around with it forking [my testing repo](https://github.com/SidhNor/bootstrap-release-grunt-test)

It contains bootstrap's `testacular.conf.js`, `package.json`, `misc` folder as well as `src` folder with several directives (just to successfully run bootstrap's default task).
`gh-pages` branch is there as well.
Installations steps are the same as for bootstrap.

To do a release just run `grunt release`.
The assumption is that git is in your path and/or you are running the git shell


Things to look into:
* safety check ensuring you are on master before running the task
* safety check ensuring your working directory is clean
* use discrete git add instead of -a
* parameters for release type (major, minor, build). I've set up the ground for that, just need to understand how and when will we bump versions from build to minor etc.
* Clean up on fail. Check what steps failed, remove tag/revert commit if need be.

I understand this is a 'nice to have but easily live without it'  type PR, so take a look when you have some spare time.