Removed glpk and linprog

As explained [here](https://groups.google.com/d/msg/julia-dev/pQEIGPv2evY/KBWJOXnWM1IJ), I moved glpk.jl and linprog.jl to packages. They're still unsupported on Windows, and GLPK can't currently use the system-installed library, so this change is removing functionality, which is why I'm issuing it as a pull request. However, Windows support should be a simple matter as soon as new Windows binaries are out, and old binaries include it.