Add single row mode of PostgreSQL 9.2

--HG--
branch : single_row_mode
Merged in larskanis/ruby-pg/single_row_mode (pull request #13: Add single row mode of PostgreSQL 9.2)
Add some more documentation to Connection#set_single_row_mode
Improve spec for Connection#set_single_row_mode
Use proper const check instead of implicit relation
Fix encoding of messages delivered by notice callbacks
Fix text encoding for Connection#wait_for_notify
Fix text encoding for Connection#notifies
Update PostgreSQL to 9.2.2 for cross compilation task