Remove dependency on System.Web for Xamarin compatibility

Hi,

This PR removes the dependency on System.Web that prevented us from using this project with Xamarin. The use of `HttpUtility.UrlDecode(str)`, is replaced by `Uri.UnescapeDataString(uri)` as an equivalent to JS `decodeURIComponent(str)`.
This issue is discussed in the SocketIoClientDotNet project:
https://github.com/Quobject/SocketIoClientDotNet/issues/6

Next steps to solve the issue would be the following:
- Publish a new Nuget package for EngineIoClientDotNet
- Update the EngineIoClientDotNet Nuget packages in SocketIoClientDotNet
- Remove System.Web references in SocketIoClientDotNet
- Publish a new SocketIoClientDotNet Nuget package

Thank you for this library!

Vincent Costel