fixed BKNDLSS-5933
rename method extendLife to expireIn and add method expireAt, with next signature

public void expireAt( final String applicationId, final String urlPrefix, final String key,
                        final long timestamp ) throws CacheServiceException
changed signature for expireAt from long to date
Merge pull request #34 from Backendless/cache_service

fixed BKNDLSS-5933 changes with expireIn and expireAt