Split package into lib and sys

This commit splits all sdl code. The ll (ffi) code is put in the sdl2-sys package, the rest remains in sdl as discussed in #227.

It compiles on `rustc 0.13.0-dev (71123902e 2014-12-29 20:19:53 +0000)`, but is broken on the laterst nightly. This is due to type inference in the audio module, I chose not to fix that in this PR.

A few things I am not fully happy about:
* The `rect` module. It could not nicely be split in its current form since the Point struct is needed in both code, and you cannot implement the methods if the struct is defined in sys
* The enum values from `video` are copy-pasted. The compiler would otherwise warn about it not being a const value. Perhaps there is an easy fix for this.
* Some modules are now only `pub use sys::x as ll`. Perhaps this could be removed and other modues could just import the required sys module.