Fix fill method typo in doc

to fill a form, method is `fill`, not `fillForm`.
Fix example test command.
Expose driver to the world.
Better naming for browser runner variables.
Fix Fill when value is undefined.

toString was called on undefined or null values.
Scenarios receive the full configuration.

Describe now expose full selenium-grid configuration content as second
argument. It used to receive only desired, but is is available in
browser.

It allows to use other drivers and configurations.
Expose config in `before` executed files.
Add some doc about how to extend driver.
Bump v0.4.0