Improve ergonomics

[Snapstromegon]

What is this feature about?

The ergonomics of this crate for users can be improved in several points to make it easier to get started and to make user code smaller.

What needs to be done to implement this feature?

• implement Default on all RequestOptions and Preconditions
• export main parts directly from lib.rs like Client or RequestOptions

What else should we know?

This would probably also simplify examples in readme and co..