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

I tested this by running the dalvik test suite as well as compile a libGDX project with it (which meant installing a locally compiled version of the Eclipse plugin, is there a better way to test this?).

I think the only "problematic" bit should be the additional parameter to `Access#checkMemberAccessible`. I couldn't get the required information from the other parameters. Is runtimeClass used for accessing shadowed members?