Use docker to run tests

[a14n]

Sometimes the user or CI doesn't not support to run tests on some platforms. It would be great if the test could be run inside its own containers (if --use-docker option is set). Thus the only requirement would be to have docker on the machine.

This would allow:

• to test against several versions of a browser
• to reduce the concerns when you try to install content_shell (or when its installation fails randomly)
• to test on platforms not supported nativelly (eg. phantomJS2 is not available on travis)

Btw as of today, we can use Docker as part of our build.

What do you think ?

[nex3]

This is a great use-case for a plugin (#49 and/or #99), but it's not something I want to put in the core test package.