Fixed round_to_str function

When ndigits was undefined, the function failed and cut all the trailing zeros in the number. This was because the ndigits == 0 condition wasn't verified when ndigits = undefined, so the resulting number from toFixed() had no decimal part, and the regular expression cut all the zeros in the integer part.
Fixed time handling in CWS

No more negative time when contest ends or starts! And the countdown is now relative to the server time and not to the local time.
Add VittGam to AUTHORS

And tidy its commits.
Clean up CWS's client-side time handling

By removing the (unused) contest_start and contest_stop parameters and
adding the (also yet unused) current_phase_begin parameter.
Some fixes to the documentation