Grunt: rename the `build-*` tasks and add new task to help package releases.

Clarifying the existing `build-*` tasks will help reduce confusion for contributors to the project by making it easier to understand when each should be run.
A new task to help the BuddyPress team package new releases will help make our release process more straightforward and hopefully save time.

The Grunt tasks are now:

* "src" -- operates on the `src/` folder. To be run during development.
* "commit" -- operates on the `src/` folder. To be run before commit.
* "build" -- updates the `build/` folder. Creates a build of BuddyPress ready for full testing (with minified assets, etc).
* "release" -- updates the `build/` folder. Creates the release version of BuddyPress, ready to be added to our distribution SVN.

In the near future, we'll probably add a `grunt watch` task which will do the same thing as the new `src` task.

Fixes #6064

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9229 cdf35c40-ae34-48e0-9cc9-0c9da1808c22