Update to spidermonkey 24

Work is being done in [mozjs-24](https://github.com/TingPing/hexchat-javascript/tree/mozjs-24) branch.

~~One major issue is that most major distros don't ship this version in their repos afaik.~~
- [x] Get it building
- [ ] Fix crashes
- [ ] Build mozjs-24 on Windows w/ VS2013
