River trends endpoint Closes #18

* Added two new endpoints to the rivers controller:
   - rivers/:id/trends/tags - For the trending tags in a river
   - rivers/:id/trends/places - For the trending places in a river

* The results are also be paginated via the "count" and "page" query
parameters. These default to 20 and 1 respectively

* DTOs for to wrap the place and tags trend data
Set the trend date in the trend DTOs

* Changed the data type of the datePublished property in
the GetPlaceTrend and GetTagTrend DTOs to java.lang.String
Order trends in ASC order of trend date