Builtin grammar types

WIP

* Generators and specs for [builtin grammar types from VoiceXML](http://www.w3.org/TR/voicexml20/#dmlABuiltins):
  * [x] - Boolean
  * [x] - Date
  * [x] - Digits
  * [x] - Currency
  * [x] - Number
  * [x] - Phone
  * [x] - Time
* [x] - Proper error handling in URL lookups