Fix missing console logs from TestCampaign constructor

After refactoring attester as a module, logger is attached too late for
the logs from campaign constructor to be displayed in the console (because
"attester.start" is raised after the initial campaigns creation).

This adds a new event "attester.config.available" which is raised at the
very beginning, before campaign creation, so we can set up the logging
early.

Close #51.
Semver and text parsing for flexible non-verbose browser cfg

Regarding the required browsers for the campaign:

The old config properties `majorVersion`, `minorVersion` and `revision`
were removed. Instead, semver-compliant version matching was put in place.

Browser config based on verbose entries with `browserName`, `os`, version
entries and `name` for display alias, was replaced with simple processing
of natural-language string (like "Chrome 27 on Android").

Close #40.
Refactor confusing browser/browsers options

Renamed `browser` to `run-browser` to avoid confusion with `browsers`
specifying required browsers for the campaign.

Close #36.