Migrate from Java 1.6 to 1.8

[paulschwarz]

The original intention was to implement a platform-independent way to build up the classpath for the compiler that is invoked during test. However, I encountered a few hairy issues and decided to approach them one by one. Therefore, this pull request can be summarised as:

• Adjust pom.xml and the annotation processor to use Java 1.8
• Add a few gitignore entries
• Add a JUnit After hook to clean up the temporary compiler output files

At this point, surprisingly all tests passed despite the issue I observed running on Windows yesterday. Specifically, on Windows clazz.getProtectionDomain().getCodeSource().getLocation().getFile() returns paths in the form "/C:/Foo/Bar/" (notice the "/" at the start of the line). My original goal was to implement a platform-independent way get this path. However, as-is, tests passed when running against a 8 JDK. So I decided not to commit my changes.

However, when I switched out my JDK to run against an 11 JDK, I had another issue and wasn't able to figure it out in a reasonable amount of time:

Caused by: java.lang.IllegalAccessError: superinterface check failed: class com.sun.tools.javac.processing.JavacProcessingEnvironment$$EnhancerByMockitoWithCGLIB$$2c456aaa_6 (in module jdk.compiler) cannot access class org.mockito.cglib.proxy.Factory (in unnamed module @0x1c72da34) because module jdk.compiler does not read unnamed module @0x1c72da34