IRestResponse instead of RestResponse

For anyone copying and pasting your example (like me) - you can get started a little bit quicker :)
Add the missed ContentLength Property
Remove redundant MSBuildBinPath import

MSBuildBinPath was replaced with MSBuildToolsPath after VS2005.
MSBuildBinPath was kept around for back-compat, but is no longer
needed. The extra import causes unnecessary build warnings.

See http://msdn.microsoft.com/en-us/library/bb397428.aspx for
a full discussion.
Remove JSON.NET from unit test project

This fixes #346
Upgraded SimpleJson to 0.25

Since SimpleJson.cs is a linked file, only one project should have a
packages.config entry for it, the one with the actual file. The others
will fail trying to update a linked file.

I removed the SimpleJson packages.config entry for all the other
projects. I left it in RestSharp\packages.config
Merge pull request #388 from Haacked/remove-unneeded-import

Remove redundant MSBuildBinPath import
Merge pull request #390 from Haacked/update-simplejson

Upgraded SimpleJson to 0.25
Merge pull request #389 from Haacked/update-jsondotnet

Remove JSON.NET from unit test project
Merge pull request #379 from hahmed/patch-1

IRestResponse instead of RestResponse
Merge pull request #381 from XixunDeveloper/master

Fixed System.Net.ProtocolViolationException issue cause of missed the ContentLength property