Fix for issue #45 - Add Until to IgnoreAttribute to ignore the test until the given date

For some reason, pushing to the pull request branch did not add to the existing pull request, so I am opening a new one and closing the original.

See https://github.com/nunit/nunit-framework/pull/69 for the full discussion.

- Merged recent changes from master into the pull request to make merging easier.
- Renamed UpTo to Until.
- The test now runs when the Until date has expired
- Added an IgnoreUntilDate property that gets set
- The date and/or time is specified in UTC. If no time is given, it defaults to midnight UTC.
- If no reason is given, it is set to "Ignoring until yyyy-mm-dd hh:mm:ssZ"
- If a reason is given, it is not altered since that information is in the IgnoreUntilDate property.