WIP: Linear programming interface

Don't commit this yet but I wanted to put it out there. This requires the current master of ``Clp.jl`` to work.

Some questionable issues:
- Choosing available solvers. Right now ``Clp`` is the default and something like ``Optim.lpsolver = Gurobi`` can be used to change the solver. 
- Where should both ``linprog`` and the standard low-level interface be documented? Currently it's at https://gist.github.com/mlubin/5154771
- Should there be a base type for solver interfaces?

@ViralBShah @lindahua @carlobaldassi