Added ability to match on any of the given timestamps

Comes in handy in various use cases where it's easy to describe the timestamps with positive statements.
For example: Reception hours could be ['9:00-12:00|mon-fri','13:00-17:00|tue-thu']
I guess you could describe it with a single positive statement and then multiple (in this case 4) negative statements, but this way it's more convenient.

The second commit changes the behavior from:
    >>> timespan.match('17:00-00:00|mon-fri|*|*', datetime(2012,12,23,0)) # Sunday
    True
to:
    >>> timespan.match('17:00-00:00|mon-fri|*|*', datetime(2012,12,23,0)) # Sunday
    False
It is true that it also makes the 00:00 end time corner case exclusive instead of inclusive, but I rather have this than having the match return true at 00:00 between Saturday and Sunday.
Perhaps we can add to the timespan syntax optional inclusive\exclusive markings... perhaps in a future commit...