More precise docs on `browsers` config option

Closes #25
Fix missing images in HTML report

If first state in suite is skipped, directory for the images
was not created and no images was shown in final report.

Fixes #37
0.4.2
Merge 0.4.2 changes into master
WIP: Don't relaunch browsers after each suite

New method of executing tests:
- launch browser
- run all suites
- close browsers only if it will not be needed anymore

This speeds up testing process, especially on public services with
queues (such as SauceLabs).

This is work in progress commit. No reporter at this time works
properly.
Replace tree reporter with flat reporter

There is no way to make tree reporter work with current runner. Default
is now flat reporter.
HTML reporter working
Teamcity reporter working