In Progress: Add caching.  (#255, https://trello.com/c/L1bLwPR9)

Default TTL is 120 seconds. Implementation: EhCache
(http://www.ehcache.org/documentation/2.5/code-samples).

See ehcache.xml for `@Cacheable`-annotated method keys or
`CacheConfiguration`.

Key methods: 
* workerService.getAll.cache
* workerService.getById.cache
* workerService.getByUser.cache