More flexible specification for business hours

[erwaller]

Currently you can only specify a pair of open and close times which apply on non-weekend, non-holiday days. It'd be useful to have open and close times which vary by day of the week, time of the year, etc.

Also, the concept of weekend days is fairly arbitrary. It probably makes sense to take this extension of the concept of flexible business hours to support days with no business hours, and have that replace the concept of weekends.

[dreaquil]

Made a pull request for a child class that should resolve the first issue described above.https://github.com/seatgeek/businesstime/pull/35