add failure_callback param

[gulducat]

Addresses issue #61

Being able to call a custom function when all retries have failed is useful for a few applications.

• Do nothing
• Log and re-raise
• Return the value of the last function call
• Send notification somewhere (email, chat, error tracker, etc)

This approach returns the last Attempt which provides access to the last function return value, or exception details depending on the failure conditions.

[gulducat]

also addresses #29