- cleanup on connection methods + documentation. custom DBAPI

- cleanup on connection methods + documentation.  custom DBAPI
arguments specified in query string, 'connect_args' argument
to 'create_engine', or custom creation function via 'creator'
function to 'create_engine'.
- added "recycle" argument to Pool, is "pool_recycle" on create_engine,
defaults to 3600 seconds; connections after this age will be closed and
replaced with a new one, to handle db's that automatically close
stale connections [ticket:274]