Add the ability to explicitly specify the path to the Java executable

This pull request fixes #32 and fixes #34.

I looked into automatically detecting the presence of IKVM, but decided against it (at least for now) for the following reasons.

1. The IKVM NuGet packages do not add a property to the project which makes it easy to detect the location where **ikvm.exe** was placed within the solution.
2. I haven't tested the output of the ANTLR Tool when run using IKVM, and I don't want the presence of the IKVM binary within the solution to automatically prevent ANTLR 4 from working, especially when the system could include a working Java binary at a standard installation location.