Convert from grunt to gulp for build

This commit converts the build system from [Grunt](http://gruntjs.com/) to [Gulp](http://gulpjs.com/). The resulting `gulpfile.js` is smaller and more expressive than the original `Gruntfile.js` and gives more control over the process as well as a performance improvement.

Note: Because the gulp runner is considerably faster, this change seems to have uncovered a race condition in two of the language tests around the new `zeroFormat` option. I will gladly help track down those fixes but wanted to start a discussion about the build conversion first.