Improved error handling and code documentation.

[ddudnik]

Hi,

This one is about error handling:

• Introduced different levels of error handling in 'make_request'. Method which handles the errors seem not that easy to read, but it does handle 3 different levels of error handling nevertheless :) In general it helps with rather important scenarios: 1. avoiding failures when API returns empty response (retry) and 2. allowing to return partial results, when client does not want to wait for too long (with 'graceful' level)
• Added logging to be used instead of 'print' statements
• Introduced EmptyResponse error
• Created unit tests for Transport class

Cheers, Denis