Pybitrix24

- Handle HTTP errors gracefully
- Add default value for `user_id` constructor argument in Bitrix24 class

- Added support for multiple Python versions (2.7, 3.5+)
- Removed any dependencies
- Improved test coverage
- Improved code stability, readability and documentation

- Merged 6 and 7. Thanks sleshery !
- Resolved issue 4

Features
- Create a `call_webhook` method to handle webhooks.
- Add a couple of exceptions handlers.

Bug fixes
- Fix 1 issue