Implement headless mode

[Huitzilo]

Firefox seems to allow a headless mode. could be useful, perhaps even faster

[Huitzilo]

Headless mode is also possible in Chrome! set options.headless = True.

Catch: Prevents the dialog to close open sessions from showing—see #5 .

[Huitzilo]

Implemented in e6d533c, but it's a bit flaky since sometimes user interaction is still required to resolve problems.

[Huitzilo]

Made new issue #7 regarding Firefox.

[Huitzilo]

Reopening since a proper implementation should also catch errors occurring with the authentication while the window is invisible. Substantial effort is required for this.