Support custom auth header and user_or_create_function

[RenderCoder]

It is not everyone will write their own UserManager I think (also am I), so it is possible to support user to change their custom user_or_create_function and auth header.

Add:

• custom user_or_create_function support
• custom auth header support Optimize:
• the README document content, user can easy to use this tool

[pimvernooij]

Thanks for this PR!

Could you have a look at:

• the test suite - all tests fail at the moment
• declare 'defaults' for the settings you add, so that existing installations can safely upgrade and configuration is not needed when running in default mode?
• avoid the use of eval() - can you find another way? Eval() is considered bad practise

[RenderCoder]

Thanks, I'll learn about it.