Serialize and deserialize week start day

[masqita]

My colleague forgot to serialize and deserialize the week start day (from_hash, to_hash) for weekly rules with an interval.

Tests added as well as documentation for the first day of week parameter.

[devwout]

My first idea was indeed to split weekst away from WeeklyInterval in its own Validation. The problem is that they are really tightly coupled. The way dates are calculated in WeeklyInterval needs to be overwritten when you take weekst into account. Furthermore, if you take sunday as the default weekst (the way it is in ice_cube), the weekst attribute has to be present in the iCal export, since the standard defines that the default weekst is monday. When you define weekst in a separate validation, the user always has the option not to include it, and the iCal export will be wrong.

About the serialization, you are right, it should be a number. We will fix that.

[masqita]

The option IceCube::Rule.weekly(2, :monday) was already implemented apparently, so I just changed the docs to use the same API as the rest of the examples. Week start is now serialized as a number.

[Coffei]

I noticed, that (de)serialization with to/from yaml doesn't support week start as well. Does this fix that issue too?

[avit]

@masquita, I rebased your changes on master to do a little cleanup (whitespace, method names, etc.) and pulled this in. The API for weekly(1, :sunday) might look a little confusing vs. weekly(1).day(:sunday), but it works.

@seejohnrun if you had any other reservations about this long-waiting open ticket, do let me know but I'll close it for now.