Jedis upgrade & custom Redis key prefixes

Hi Alex,

thanks for the great component!  I came across a runtime error when I upgraded the Jedis lib to the latest version (v2.4.2). The latest Jedis uses the common-pooling2 lib instead of the old one and  there are bytecode signature conflicts.  Unfortunately, in order for shiro-redis to work correctly it needs to be re-compiled with the latest version of Jedis.  Otherwise if a 3rd party component uses a newer version of Jedis (> 2.0.0) and shiro-redis is built with the old one, things get screwed.

Another thing I added is the ability for the integrator to define custom Redis key prefixes for the session/cache entries.  This is required in order to keep a clean key hierarchy under our own namespaces.

Finally, I added a sample JUnit test for future use and I'll add some tests for the existing code as well.

Cheers,
Chris