DATACMNS-525 - Add KeyValuePersistenceExceptionTranslator, fixing Todos.

Added key/value specific PersistenceExceptionTranslator and use it within KeyValueTemplate. Fixed some todos in JavaDoc and added missing tests. Renamed test class for SimpleKeyValueRepository according to the changed name.

Original pull request: #95.
DATACMNS-525 - Removed Hazelcast implementation.

Original pull request: #95.
DATACMNS-525 - Removed EhCache implementation.

Original pull request: #25.
DATACMNS-525 - Introduced EhCache repository implementation.

Original pull request: #95.