Update .NET client to Search API v3 [$50]

[roback]

Update the client to use the new Blog Search API v3.

Notable changes from the previous version of the Blog Search API are listed under https://developer.twingly.com/resources/search#changes-from-previous-version.

The ruby client was updated in twingly/twingly-search-api-ruby#65, which can be used as help. Some XML fixtures can also be copied from there if needed.

If possible, try to keep all classes/methods/properties backwards compatible with the previous version of the client by marking methods as deprecated. Similar to what was done in the Ruby client.

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There is a $50 open bounty on this issue. Add to the bounty at Bountysource.

[dentarg]

If you are considering solving this issue, you might be interested in also solving one or more of the following issues (they also have bounties from https://www.bountysource.com/)

• Update PHP client to Search API v3 [$50] twingly-search-api-php#22
• Update Java client to Search API v3 [$50] twingly-search-api-java#29
• Update Python client to Search API v3 [$100] twingly-search-api-python#31

[dentarg]

@amironoff interested in the bounty here?

[dentarg]

Register (for free) at https://app.twingly.com/ to get an Blog Search API key to use when developing and testing (make sure it isn't recorded in any fixture)