Removed JSONP callback parameter + related hardcoded string magic, added support for date/time limiting board searches

For more reliable (and standardized) JSON decoding of responses, I removed the callback parameter that were sent with all requests. This parameter creates a JS function scope around the response data, which is supposed to be passed to a browser handler when done, in some capacity.

When this was removed, the additional substr massage of response data went along as well.

As a bonus I also added parsing of datetime parameter in the board method.

This pull request should be updated with the latest HEAD from ltworf/master.