Move ffi into a seperate crate.

[jdroenner]

#25: moved ffi to a new crate: gdal-sys.
I also added a cargo workspace where the rust-gdal crate is the root and gdal-sys is a member.

[frewsxcv]

It looks like Travis found some errors when running cargo test. Some of the imports might need to be changed within mod test?

[jdroenner]

It appears that the problem is use gdal_sys::gdal; in the raster mod and use gdal_sys::ogr; in the vector mod.
I'm using use raster::{gdal, Driver, RasterBand}; in the other source files of the module which works with the nightly version of rust but not in stable and beta. What is the correct behavior? Using pub use ... in the module works fine with beta and stable but i don't think that the ffi should be public in rust-gdal.

I will put use gdal_sys::gdal; and use gdal_sys::ogr; directly into the source files to fix this.