Add definitions for Greasemonkey API

This pull request adds `greasemonkey/greasemonkey.d.ts` which defines global functions and global variables provided to user scripts running on Greasemonkey.

Greasemonkey API document is here: http://wiki.greasespot.net/Greasemonkey_Manual:API

Because Greasemonkey became popular first, this kind of extensions such as Tampermonkey for Google Chrome or Ninjakit for Safari, they all have compatible API as Greasemonkey.