Added support for standard LP interface

This pull request does the following:
- Expose add_rangeconstr! and add_rangeconstrs!
- Creates a new file that defines the standardized interface

You can now use Gurobi from MathProg by running MathProgBase.setlpsolver(Gurobi)

Still need to finish support for problem modification, but this will probably require more work on the basic C interface as well to expose some more functions nicely.

(Could you give me commit permission so I can keep tweaking this?)