deprecate **adapter: jdbc** and **adapter: jndi** configurations separately
do not advocate for using **adapter: jdbc** in the README, also AS400 is deprecated
remove (what seems to be pretty) dead-code (method not used in AR >= 2.3)
add some API doc
remove DB2's (no-longer used) as400? method
rename `with_connection_retry_guard` to simply `with_jdbc_connection`
make sure we do not throw (checked) NamingException from a JRuby method
update with-connection to handle connection retrieval as well as transient failures :

- we're now retry driver.getConnection as well as data-source lookup
- transient SQL failures are retries on the same connection
- only recoverable SQL failures are retries on a new connection

.. mostly assuming JDBC 4.0 compilant (Java >= 6) drivers