AES-GCM cipher support where available

The initial encryption drop only had AES-CTR support because authenticated modes such as GCM are only available in Java 7 and up, and our trunk at the time was targeted at Java 6. However we can optionally use AES-GCM cipher support where available. For HBase 1.0 and up, Java 7 is now the minimum so use of AES-GCM can go in directly. It's probably possible to add support in 0.98 too using reflection for cipher object initialization.