Refresh Token, Web/Public App support (salesforce-marketingcloud#116)

* Implemented RefreshToken, Public-Web app support for OAuth2.0

* Added unit tests and other small changes

Added unit tests for create_payload method, added venv in .gitignore, block of code for setting redirectURI was misplaced

* Added a unit test, fixed bugs

Added the test_authToken_should_differ_if_refresh_token_is_enforced unit test, fixed two bugs in client.py

* Updated the README.md file

* Renamed a test method and added a class docstring for the test class

* Addressed code review requested changes

* Updated SDK version from 1.2.0 to 1.3.0

Updated SDK version from 1.2.0 to 1.3.0 in FuelSDK/__init__.py, client.py, rest.py and setup.py

* Added tests config

* Small changes in FuelSDK/Public_WebAppTests/test_ET_Client.py

Replaced the setUp with setUpClass and updated the docstring

Updated setup.py to conform with twine rules for the readme.MD file

Merge pull request salesforce-marketingcloud#48 from knight-dj-et/master

Changing version

added requirements and install instructions

Initial Beta Release