Clean up core text / core graphics bindings

[brson]

Some of the CG bindings are in rust-cocoa. They should be moved to rust-core-graphics, and rust-cocoa removed from servo. quartz_native_font contains some core text bindings that should be moved to rust-core-text.

They all need high-level bindings that manage destruction properly.

[pcwalton]

CTFonts look like Core Foundation objects (they use CFRelease). The rust-core-foundation API objects help manage the lifecycle of these properly.

[burg]

Where possible, we should mirror bindings after the divisions in native APIs. So, one for CG, CT, CF---with higher-level help in rust-core-foundation.

[burg]

I created rust-core-graphics to hold the CG bindings already used, and will next make rust-core-text. Then, we can move the remainder of rust-cocoa to rust-core-foundation and remove rust-cocoa. Then, rust-cg and rust-ct can be given high-level bindings incrementally.

[burg]

Followup bug #152