Update for jdk1.7.0 >= u25: use CallerSensitive annotation

I had like many others problem running grails applications using the latest jdk version.

As I still have problems with the fedora openjdk (based on and early icedtea 2.3.10), I tried to replace the getCallerClass(depth) calls with the new CallerSensitive annotation (see http://openjdk.java.net/jeps/176).

This pull request is a proof of concept that breaks compatibility with older jdk releases, but can be useful for future jdk updates.

Tests seems to pass and grails applications are running correctly.