Add error_code to DeliveryError

DeliveryError (and its derived classes) now get a 'error_code' property, set with the error code received from the server. This can be used to differentiate between various errors that all fall into InvalidMessageError, for example.
Merge pull request #20 from pazustep/error-code-in-exceptions

Add error_code to DeliveryError
Update .gitignore with .rvmrc.
Get rid of ruby-debug gem.
Get rid of ruby-debug gem.
Retry requests on Timeout::Error.
Remove unused constant.
Add DeliveryError for timeouts.
Just use relative paths instead of helper method.
Remove .bundle/config.

There's no reason for this to be part of the repository, and it may
differ for each user based on individual preferences and configurations.
Merge pull request #22 from tylerhunt/retry-timeouts

Retry timeout messages.
Merge pull request #23 from tylerhunt/remove-bundle-config

Remove Ignored .bundle/config
Merge pull request #24 from tylerhunt/relative-requires

Use Relative Paths Instead of Helper
Prepare for 0.9.13 release.
Parse MessageID and Subject from the Bounce API response.
Prepare to 0.9.14 release.
Save a received MessageID in message headers.
Load 'json' gem in both Mail and TMail examples.
Remove empty development section from Gemfile.
Add #with_retries method.