fixed issue #20, check that access target is not in boot classpath

Second try, not sure what i was thinking on the first try. Dalvik tests are running through. Target and caller need to come from the same classpath in all three methods of Access. To be able to check this, i had to modify a few parameter types and introduce a new one for `checkMemberAccess`.

Note: i added `ClazzInfo#getClazz()` which may be undesireable. To my understanding, it may return null if the ClazzInfo is a phantom class. I'm not 100% sure if my handling of that case is valid in checkMemberAccess. If not, could you elaborate why?