Implement case sensitive property matching

[SakuraIsayeki]

This PR resolves #815, by implementing a new serializer option, named PropertyCaseSensitivityHandling:

public enum PropertyCaseSensitivityHandling
{
    // Case-insensitive matching (default).
    CaseInsensitive = 0,

    // Case-sensitive matching.
    CaseSensitive = 1
}

To preserve continuity with prior implementations, default is set to CaseInsensitive.
With the new behaviour provided by CaseSensitive, we can now have differentiation and validation for casing, which is required by several API specs (Binance & xAPI, to name a few examples).

As this behaviour has an extremely lightweight implementation (only one extra check on each Property matching), it does what it says on the tin, and only that.

Most of the matching with convention-following PascalCase C# properties will fail if you don't add [JsonProperty("name")] to each property, or write some esoteric conversion between JSON standard camelCase (or whichever casing is used) and C# Properties.

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A few footnotes...

This PR was inspired by the stale #601, where I factually stripped the Casing implementation to form my own.
Furthermore, as this is my first ever PR to Newtonsoft.Json (hello guys!), I might've inadvertently broken a few long-standing conventions along the way, so any feedback will be most appreciated.

[pierregarcia]

Really glad to see a PR about that. We had to fork the repo to make these changes. It would be nice to see them integrated in Newtonsoft