Adjusted working with requests

[krasoffski]

Code adjustments before attachments implementation.

• Python requests version 2.4.2 introduced json argument. Thus, no need to specify Content-Type: application/json with json argument.
• Increased required minimal Python requests version to 2.4.2 as it was released more then 2 years ago.
• Clean up warning for __init__.py files for flake8.
• Fix length of doc string up to 79 chars.
• Added Bearer auth to session scope to avoid useless arguments each time for performing requests.