OnTime name change

https://github.com/bugsnag/bugsnag-notification-plugins/issues/90

- I've changed all the config.json references to OnTime, and double checked that they still exist in Axosoft.
- I've updated the icon.png to contain their new 'leaf' logo.
- All the references to OnTime have been replaced with Axosoft.

No screenshot - nothing interesting changed :smile: