Add Javascript scripting support using MuJS

Implements JS with almost identical API to the Lua support, except:
- The global mp, mp.msg and mp.utils are always available
- On error return undefined and the error is at mp.last_error_string
- Timers are JS "standard" global set/clear Timeout/Interval/Immediate
- Supports CommonJS style modules/require
- At mp.utils: read_file (any), and load_file, run_file (for js files)
- Global print and dump functions
- mp.options currently not supported

Requires MuJS (clone, make, make install):
- http://mujs.com/ - git://git.ghostscript.com/mujs.git
- Mirror - https://github.com/ccxvii/mujs

Note: MuJS is a new project and this is probably its first major use outside
of MuPDF. Please keep an eye open for bugs.
I'll look at the patch and update the code.

FWIW, following a conversation we had some time ago where you mentioned that I most likely don't need to use suspend/resume like the lua code does, my code indeed doesn't use any of those, and as far as I can tell, I never bumped into cases where I'd have use for them.

I still left them at the code in case scripts need them, but never played with it more than invoking them once or twice to validate that they seem to work.