Support for grunt 0.4

This makes grunt-bump compliant with grunt 0.4, by removing the use of the helper and placing it in its own module, and switching to grunt-contrib-nodeunit instead of the old built in test task.

Currently, it's set to use grunt 0.4.0rc4 - perhaps we should wait till 0.4 final to merge? Also, after merging, this would ostensibly require a version bump to grunt-bump itself.