class: remove throwDB and use MonadThrow from exceptions instead
replace MonadBaseControl IO with MonadMask where applicable
generic MonadDB instance: lift folds using MonadTransControl
connection: fix random occurences of PGconn double-free
connection: create data type for connection data
examples: add simple catalog of books
monad: remove MonadCont instance for DBT
class: remove Functor dependency as Applicative implies it
monad: do not wrap ExitCode in DBException as that alters exit code of the application