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
Update README

* Improved the description of the SwiftRiver API; what it is and its main functions

* Information on where to find documentation about the API, where to ask questions,
authors and licensing
Merge pull request #4 from ushahidi/readme-update

Readme update