When retrieving runtime in GC callback, don't require a green task.

This means that when other runtimes are present (for example an
instrumenting runtime wrapper for profiling) this code doesn't fail.
Merge pull request #112 from glennw/allow-other-rt

When retrieving runtime in GC callback, don't require a green task.
Fix JSVal::is_boolean
Merge pull request #113 from mukilan/fix-is-boolean

Fix JSVal::is_boolean
Turn off the awful warning stream.
Add a non-opt, debug feature.
Allow the user to set their own GC callback
Merge pull request #115 from kmcallister/gc-callback

Allow the user to set their own GC callback
Merge pull request #114 from jdm/features

Add a non-opt, debug build feature.
remove managed_boxes, static => const
Merge pull request #116 from Jurily/master

remove managed_boxes, static => const; r=Ms2ger
rust update for libgreen dependency.
Merge pull request #117 from servo/rustup_20141105

rust update for libgreen dependency.
Fix warnings with newer Rust
Merge pull request #118 from metajack/warning-police

Fix warnings with newer Rust
Remove GC callback, it crashes with rust upgrade. jdm says it's not required.
Merge pull request #119 from servo/rustup_20141105

Remove GC callback, it crashes with rust upgrade. jdm says it's not required.