Add TimeRange class

TimeRange class added to handle adding a date to two times and adding
the correct date even when the time should roll over to a new day.

The primary use case for this class is when you have opening hours and the closing time might roll over into the next day.

For instance:

```
opening_hour = '17:00'
closing_hour = '03:00'
date = '2013-12-24'

timestamp_opening = '2013-12-24 17:00:00'
timestamp_closing = '2013-12-25 03:00:00'
```