Support custom json object in JS client

- Added connection.prototype.json which is initialized as window.JSON
- Changed default ajax content type to text
- Added connection._parseRespons helper to parse ajax responses from text to JSON (when it's not JSONP)
- Update all JS client code to use connection._parseResponse
- Added unit & functional tests
- Updated affected tests

#2654

Conflicts:
	samples/Microsoft.AspNet.SignalR.Hosting.AspNet.Samples/Scripts/jquery.signalR.js
	samples/Microsoft.AspNet.SignalR.Hosting.AspNet.Samples/Scripts/jquery.signalR.min.js
	samples/Microsoft.AspNet.SignalR.LoadTestHarness/Scripts/jquery.signalR.js
	samples/Microsoft.AspNet.SignalR.LoadTestHarness/Scripts/jquery.signalR.min.js
	src/Microsoft.AspNet.SignalR.Client.JS/jquery.signalR.core.js
	src/Microsoft.AspNet.SignalR.Client.JS/jquery.signalR.hubs.js
	src/Microsoft.AspNet.SignalR.Client.JS/jquery.signalR.transports.common.js
	src/Microsoft.AspNet.SignalR.Client.JS/jquery.signalR.transports.longPolling.js
	tests/Microsoft.AspNet.SignalR.Client.JS.Tests/Scripts/_references.js
	tests/Microsoft.AspNet.SignalR.Client.JS.Tests/Scripts/jquery.signalR.js
	tests/Microsoft.AspNet.SignalR.Client.JS.Tests/default.html