Added support for passing in the path to the java.exe

My build machine (which I don't control) does not have Java installed. As a workaround, I can include my own Java virtual machine into my project. In my case, I'm including IKVM via NuGet. Antlr4ClassGenerationTask assumes though that the Java installation I want will be listed in the registry. In my case, it isn't. I've added a JavaBinPath property to Antlr4ClassGenerationTask which, if you use the targets file, will take the value of the Antlr4JavaBinPath property. If JavaBinPath is set, Antlr4ClassGenerationTask uses that value as the path to the JavaVM. If JavaBinPath is null or empty which is the default, Antlr4ClassGenerationTask uses JavaVendor and JavaInstallation as it did previously.