GeoDistanceFilter

Hi Alexander,

My name is Michael Wallett, and I work for Opentable UK. There is a big push at Opentable to improve the search capabilities of our site, and I'm very impressed with PlainElastic.Net. I love your glass box principle, it makes total sense when it comes to building clients.

We have a team of developers who would like to contribute to your project, myself included.  Our short term goals would be to fill out missing features and expand the tests. Our immediate need is to work with geo queries, filters, and sorting. I've implemented geo  filtering for this pull request.

Please review my code and give me feedback. If you want me to make changes let me know and I will get it sorted. If you are keen to work together that would be great!

Thanks,
Mike