Complete implementation of VarArrayBuffer

* Implemented PPB_VarArrayBuffer_{Create, ByteLength}
* Implemented glue.structToJSVar for ArrayBuffer
* Added ArrayBuffer test to ppapi_messaging
* Added ArrayBuffer test to ppapi_vars
* Added IE10 polyfill for ArrayBuffer.prototype.slice
Add AUTHORS and CONTRIBUTORS files.
Fix nacltest.js on IE and note erratum.