Dropped Json.NET dependency

Json.NET is much faster than DataContractJsonSerializer. However, this
library is about making web requests... The time saved by using Json.NET
is insignificant compared to the rest of the process. So, it's better to
drop the dependency. If someone really wanted to use Json.NET it's a
super basic IRestDataSerializer implementation. They can pass that
IRestDataSerializer into their RestClientBase class's constructor.
Added Readme file