Adds ILP solver ENV checks for GUROBI and CPLEX

[maxkratz]

Closes #202.

I've added ENV checks for GUROBI and CPLEX and tested them both successfully on my Linux machine. Please check (and confirm) that this also works on your Windows system, @arg0n1s.

• GUROBI has publicly accessible (static) fields for the version (major, minor, technical).
• CPLEX is also capable of returning the version number but only if the JNI can access the native libs (which would not be the case if the ENV setup is wrong). Some of the CPLEX classes contain a private static final field for the version; therefore, I've implemented the Java version request via reflections.
• GLPK does not need any special ENV setup (at least on Linux), so I did not implement any checks for it.

[maxkratz]

By the way: It seems CPLEX development has staled: https://or.stackexchange.com/questions/12222/is-there-any-information-on-why-ibm-is-not-releasing-a-new-version-of-cplex