[issue #12] initial implementation of a simple KV-cache that keeps itself clean
[issue #12] additional constructor and simple jdoc
[issue #12] added test for TimedSoftReference
[issue #12] added javadoc for TimedSoftReference
[issue #12] added lock upgrade to properly initialise the cache
[issue #12] TimedSoftReferenceTest jalopy format
[issue #12] added test for PurgingCache and corrected all errors that were discovered by it
[issue #12] added purge interval check and corresponding tests
[issue #12] javadoc for PurgingCache
[issue #12] added sync for the keyPurgeInterval getter and setter to ensure consistent behaviour on concurrent access
[issue #12] disabled null key for PurgingCache
[issue #12] minor comment
[issue #12] widened range of allowed get parameters
Merge branch 'feature/12-purging-cache' into dev