Add open-source version of gurobi optimizations

[skent259]

Currently methods 'mip' and 'qp-heuristic' for mismv() and mildsvm() are only implemented in Gurobi, which is not open source. Consider adding an option to have these in an open-source implementation, if possible.

Related to #8

[skent259]

Note that this is important for users with large data sets and without access to Gurobi, since misvm(..., method="heuristic") does not have a pre-computed kernel matrix option.