mathprogbase nonlinear interface

Just wrote up a preliminary MathProgBase interface for NLopt. There are some minor tweaks I needed to make to NLopt.jl for vector-valued constraints (there was an inconsistency with passing an empty vector vs. a matrix -- I pass an empty matrix now). It's functional to the extent of passing a basic test. Still missing is support for the various solution parameters and documentation. Any comments, @stevengj?

Ref #17