Fix dozer field mapping error. Closes #2

* Removed the image field mapping between the Drop entity and GetDropDTO
The source and destination fields in the mapping are incompatible
Restore image field mapping in dozer config

* Restored the initial mapping and instead changed the data type
of GetDropDTO#image to java.lang.string. There is no need to
encapsulate the url of the drop image in an image object
Fix invalid parameter index

* The dateAdded parameter is at position 3; not 2
Merge branch 'fix-dozer-mapping-config' into solr-search
Search endpoints for each resource

* Each of the 4 main resources; drops, rivers, buckets and accounts
have their own endpoint in the search controller. The basic parameters
(q, count and page) are the same for each of the endpoints.

* TODO Add date filtering for drops search
Background drop indexing

* Background drop indexing is performed via a scheduled
task that uses indexer.properties to fetch the drops that
are yet to be indexed

* TODO: Ensure background indexing doesn't clash with realtime
indexing - synchronize access to method that updates the ID of the
last drop to be indexed
Allow both max_id and since_id query params. Closes #1

* When querying river/bucket drops, the max_id parameter
was being ignored if the since_id parameter was present. The fix
allows both parameters to be applied when querying drops

* Single DropFilter class - Decoupled the DropFilter class from the River
and Bucket DAOs since the filter parameters are largely the same
Change drop indexing strategy

* Only use background indexing strategy to avoid complications involved in
setting the ID of the last drop to be indexed

* Added bucketId and riverId to the list of fields to be indexed. In this way, we
can perform full text search against a specific bucket/river.
DropDocumentRepository has methods to facilitate this.

* NOTE: Solr 4.0 has an issue with adding multiValued fields via the API. This
issue has been fixed in Solr 4.1 onwards.
See https://github.com/SpringSource/spring-data-solr/pull/1 for more details