Use custom converter for Enum types 

- Based on http://anismiles.wordpress.com/2010/08/04/postgres-enum-with-hibernate/ 

Search now works with http://localhost:8080/trialverse/concepts/search/findByType?type=DRUG
Rename to more generic EnumConverter

- EnumConverter takes a parameter for the enum to convert
Rename route for findByType
Reorganize model/repositories:

 * Repositories in .repositories
 * Move EnumConverter to common-hibernate
Minor improvements to data mapping

 - Add CodeSystem
 - Proper @ManyToOne on concept.code_system and study.indication
 - Pagination for concept search