Set channel_id when adding new river drops

* Added entity mapping between a RiverDrop and Channel

* Update channel drop count when a river drop is deleted. We also now
check for permissions before removing a drop from a river

* Added mapping class mapping in the dozer configuration. When creating
new drops, the mapper did not now how to handle the "original_url" property.
This was not getting caught by the tests because this property was not
being specified

* Removed the "channel" column from TABLE "rivers_droplets"

* NOTE: This branch was created by branching off "solr-search"
* Closes #11
Validate the riverIds and channelIds drop properties

* For the drops to be submitted for posting to the DB, these
properties must be specified. Refs #1
Set id in ChannelUpdateNotification objects
Rectify exclusion of existing river drops

* The algorithm for excluding duplicate river drops was flawed
It appears Map is entries are not mutable. Therefore, for predictability,
remove entries and modify them then add them back (to the Map)

* Extended the length of the tag and tag_canonical columns to 100 chars
Fix river_channels drop_count update

* The parameter values for the drop_count update query were
wrong.

* Convert the channelDropCount to "long"  before incrementing.
Incrementing java.lang.Long objects has undesirable side effects
and as such, the drop_count for some channels was constantly at 0
despite the channel having drops in "rivers_droplets"
Accessor & mutator methods for dropCount

* The drop count for the river channels was not being
set so zero value was being sent to the client