Added function: JsonRequest.JsonGet

- Providing a simple, oneliner to make a webrequest and return the raw json response as string
- Added dependency package: Newtonsoft.Json
Updated NuGet packages

- Updated package: FluentAssertions
- Updated package: NUnit
Added function: JsonExtensions.IsValidJson

- Function determines if the provided raw json string is valid, or not.
Added package "Newtonsoft.Json" to Tests
Merge pull request #3 from hanssens/json-helpers

Merging "json-helpers" into "vnext"