Use Firefox -headless

[fluffysquirrels]

Discussed in #14.

Branch: ff-headless

Sub-goals:

• Travis tests should use it
• Documented example with -headless
• Documented example on user's desktop
• Documented example with an xvfb session

[fluffysquirrels]

3ae27df is a test commit onto branch ff-headless for this. It's a change on top of equalsraf's chrome-tests branch, which has the custom capabilities code.

[fluffysquirrels]

The latest Travis build on the branch just passed, and I'm thinking of merging it soonish.

[fluffysquirrels]

Semi-duplicate: #3