This allows multiple month days to be used in the :on option

```
Recurrence.new(:every => :month, :on => [15, 31])
```

- Separating monthday and weekday logic into modules to make it easier
  to understand the flow
- Adding tests
- Shift is disallowed for multiple :on days

…thomas07vt/recurrence into thomas07vt-issues/21-multiple-monthly-days

# Conflicts:
#	README.md
#	lib/recurrence/event/monthly.rb
#	lib/recurrence/namespace.rb