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
Update changelog and readme
Merge new browser launch model into master

Browser are now started only once and kept open during all tests run
time. This greatly reduces latencies and speeds up the tests.