add grunt-release to automate releases

see grunt-release documentation at its repo: https://github.com/geddski/grunt-release

Added the bare minimum amount of configuration for the gruntfile.

quick overview: grunt-release bumps the version number in package.json, adds, commits and tags the change in git, pushes to the default configured remote, and publishes to npm, all in a single command

key points:
`grunt release` or `grunt release:patch` increment the patch number
`grunt release:minor` increments the minor version number
`grunt release:major` increments the major version number

Plenty more config options available