Time plugin: time range queries (#386).

The Time API provides methods for retrieving topics and associations based on a created/modified time range.

Time service:
    - Collection<Topic> getTopicsByCreationTime(long from, long to)
    - Collection<Topic> getTopicsByModificationTime(long from, long to)
    - Collection<Association> getAssociationsByCreationTime(long from, long to)
    - Collection<Association> getAssociationsByModificationTime(long from, long to)

REST API:
    - GET /time/from/{from}/to/{to}/topics/created
    - GET /time/from/{from}/to/{to}/topics/modified
    - GET /time/from/{from}/to/{to}/assocs/created
    - GET /time/from/{from}/to/{to}/assocs/modified

Note: these methods rely on the timestamps and property index recently introduced (4.1.1-SNAPSHOT).
Topics/associations existing in a DM 4.1 installation have no timestamps.

See #386.