Every n days months years support

Hello James Still,

I am a .NET developer. I had forked your repository and added few more features to the ScheduleWidget by creating a branch from your `AnniversayEvents` branch. Now the widget supports following options for each frequency except `Quarterly`.

Daily
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1. Every n days

Monthly
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1. Every n months, selected week(s)/every week (1st week to last week), selected week day(s) (Sun to Sat)
2. Every n months, selected day of month, selected week day(s) (Sun to Sat)

Yearly
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1. Every n years, selected month, selected day

These features were initially considered as the requirements given by Mr. Dan Sargeant https://github.com/danspam) to complete the scheduling functionality for his product. He suggested me to use your ScheduleWidget as it has most of the options that we require and advised me to extend it for meeting our requirements. I added the new code and it works great for us. Then we decided to let you know about these changes and add these options to your original ScheduleWidget version if you think it is useful.

I have reused the property 'FirstDateTime' of `Event` class to complete the above features. I think it has been added to provide `every n weeks` feature for ScheduleWidget by Dajo Hein (https://github.com/dajohein).

I had also seen one issue raised 8 months ago in your repository to support specific day of month (https://github.com/jamesstill/ScheduleWidget/issues/2). This option is also available with the new code.