Scheduler based on mongo

Hi guys!

Unfortunatly, we don't have sql-based db in other projects, so we have to port your scheduler to mongodb:)
Here is a simple implementation, may be not as perfomance as possible, but we use it in production as well.

Some notes about implementation:
1) I have to split scheduler.mongo solution into two parts: core and executable. The reason is following: I need to reuse core part, but i don't like to use topshelf and store settings in app.config. 
2) The basic idea is explained here(https://blog.serverdensity.com/replacing-rabbitmq-with-mongodb/).
In addition, I use ttl sparse index on PublishedTime to purge items from collection.