Create Tutorial that outlines most important CEF features. Create tutorial.py example.

[cztomczak]

Draft is here: https://github.com/cztomczak/pycef/blob/master/docs/Tutorial.md
Tutorial code will go to examples/tutorial.py

Important notes:

• Add a note in tutorial that client handlers often change during updates to new CEF, thus to minimize required code changes one may use **kwargs in handler callbacks so that when a new param is added it won't break app. However a proper solution in the future will be to implement handlers as classes/interfaces that would allow for validation of func signatures in an automated way - Issue Expose handlers as classes/interfaces: ClientHandler, DisplayHandler, LifespanHandler, etc. #215.

[cztomczak]

Tutorial was completed: https://github.com/cztomczak/cefpython/blob/master/docs/Tutorial.md

[cztomczak]

New sections added to Tutorial:

• Change user agent string
• Off-screen rendering