Numerous stability improvements and bug fixes.

Fixed a bug causing Rust's borrow checker to malfunction when mruby.r…

…aise was called. Removed mruby.raise.

Changed GC'd Rust types to live in Rc<RefCell<T>> and added &mut T op…

…tion to method definition macros.

Fixed a bug where classes defined under another class or module won't…

… get redefined.

Added the possibility to define methods on mruby primitives.

Made Module public.

Added missing Class & Module functionality. Added pure mruby class de…

…finition. Added instance variable handling.

Fixed a couple of issues concerning mruby errors.

Removed mruby build dependency & fixed a couple of bugs.

Added a simpler way to implement MrubyFile with the mrclass! macro an…

…d added panic capturing in mruby, along with many other improvements and fixes.

*Note:* Current version requires Rust nightly until Rust 1.9.0 is released.