README.md: Updated testing section and referred to it in the contribution section.
README.md: Fixed typos.
Week selector rework. Using ISO week dates. Closes #46.

Not sure what the week selector was doing before this rework. I asked
the author but it looked that he does not know either (at least when I
asked, a couple of years after he wrote the code) …
https://github.com/AMDmi3/opening_hours.js/issues/18

See https://en.wikipedia.org/wiki/ISO_week_date
demo.html: Removed wrong code snipped added in c33f909f742f5520a5865186961242f31a4034a1.
Wrote test to check if rule with closed|off modifier is additional. Related to #53.

Test is not enabled yet because of another bug (#50).
Added source of source code snippet.
Removed debugging code and commented out snippets.
Throw error if week number is not in range (1-53).
Clean up of parseWeekRange.
Simplified week_stable calculation.
Created icon. Related to https://github.com/simone-f/qat_script/pull/5#issuecomment-53637861.
Reverse week ranges are currently not supported …
Week selector comes after month selector(s).

* https://en.wikipedia.org/wiki/ISO_8601#General_principles
* https://wiki.openstreetmap.org/w/index.php?title=Template%3AKey%3Aopening_hours%3Aspecification_table&diff=1077977&oldid=1077762
Reworked parseMonthRange.
Do not generate warning for `open "test"`. Might be false positive.
Node that possibility of a comment in the warning.
Fixed evaluation of `week 1-53/2`.
Cleaned up parseWeekRange and added warnings if period is out of range.
Found better name for function.
Fixed evaluation of value `Jan 31-Feb 1,Aug 00:00-24:00`.

If the monthday and the month selector are used in conjunction,
separated by comma (instead of space) then they are both combined into
one selector and are not two separate selectors which would have to
match both in order for this rule to be applied.