jasmine tests and extended array buffers util code

This is intended to help provide a template for writing tests in Typescript with Jasmine. 

* added `hexStringToArrayBuffer`
* added typescript definition interface file for `ArrayBuffers`
* added third_party/DefinitelyTyped directory with README
* added typescript DefinitelyTyped jasmine interface files to third_party
* added jasmine tests for `ArrayBuffers` code. 
* updated Gruntfile to run new tests

TESTED: grunt test