Initial GC implementation

Merge branch 'master' of https://github.com/mcintyret/jvm into gc

Conflicts:
	src/main/java/com/mcintyret/jvm/core/Heap.java
	src/main/java/com/mcintyret/jvm/core/exec/ExecutionStackElement.java
	src/main/java/com/mcintyret/jvm/core/opcode/invoke/Invoke.java