Add ability to create a compartment with a provided global.
Merge pull request #34 from jdm/newglobal

Add ability to create a compartment with a provided global.
Correctly define convert field of JSClass as an extern 'C' fn, not a *u8. Add a global gc function to trigger a gc.
Merge pull request #36 from jdm/foopy

Correctly define convert field of JSClass as an extern 'C' fn, not a *u8...
Warning police.
Upgrade Rust.