Validation method?

Currently it does not seem like there are any methods to check if a string can be parsed (using ```TimeOfDay.parse(...)```). As a result, the only way to check if a string is parsable is to catch an Exception, which is less than ideal.

I think it would be beneficial to have a ```TimeOfDay.is_parsable?(...)``` method to solve this, and I'm willing to submit a pull request for it.