DATAREDIS-248 - Fix generics in DecoratingStringHashMapper to be Java 8 compatible.

Original pull request #13.
DATAREDIS-245 - Prevent Excessive Thread creation in tests.

Due to some missing cleanup procedures excessive thread creation lead to JVM crashes during a full test run on some machines.
Improved test harness in order to properly shutdown unused threads.

Added idea artefacts to .gitignore.
DATAREDIS-243 - Ensure compatibility with Spring Framework 4.0.

Updated dependency of XStream since Spring framework 4.0 requires a newer version.
Spring 4 added another method to get cached object to the Cache interface. The method has been added to RedisCache in order to provide compatibility, while calling get internally and casting values to required type.