Modified Gesv to accept nil pivots and complex matrices

Hey, a few minor modifications:

I've let Gesv() accept a nil argument for ipiv according to the spec.
Gesv() will also now call zgesv() if the arguments are complex.
I didn't add this for other functions e.g. Gels().
The symbols cdotusub and zdotusub weren't found on OS X.
Added linux/darwin switches for cgo.
Removed dependency on atlas.
Ran gofmt.

Let me know if you'd like only a subset of these.