PostgreSQL 2.0.0 Beta 4

Changes:

• Moved all methods from Database to Connection, leaving Database to be a simple class containing all the information to connect to a database.
• Updated PostgreSQLError and replaced all uses of DatabaseError with it.
• Created method for performing multiple queries in one transaction, using a specific isolation level.
• Added option to the listen function to stop listening.
• Prevented the listen function from crashing with a fatal error. Now an error is returned instead.
• Refactored all binding code into a similar structure as Vapor MySQL.
• Implemented handling of Date, instead of parsing the date into a string.
• Added unit tests for the new date type parsing.