Added support for a custom handler, called when toobusy is triggered
Pass the next parameter to the handler
Made  be the last argument
Created tests for the handler callback
Updated README with  description
Added test case for using both a handler and options
Added option to change the status code sent when the server is too busy
Updated version to 0.1.5
Merge pull request #6 from mkretschek/issue-5

Added a `handler` parameter (resolves #5)