Pyzendesk

- Reverted https links to http
- Renamed requirements_travis.txt to requirements_ci.txt
- Updated dependencies
- Added icons

- Added class Agents to get agents' information
- Added class Admins to get admins' information
- Moved samples into the package
- Replaced setup.py with setup.cfg for build

- Added Users Api

- Added method get_custom_field
- Added method get_requester_email
- Added method search_all

- Added Tickets Api
- Added Attachments Api

- Added dependencies for setup.py