Refactored database connection setup/close code into to a singleton class (DBConnection)

This plugin is awesome. I have refactored the database connection setup code present in the DashboardBuilder class into a singleton class DBConnection. 

I think this will make it easier to manage the DB connection as this class can handle the instantiation and closing of the connection. 

This has been tested successfully on Jenkins version 1.590.

Should the pom file be updated to reflect the latest Jenkins version?