Fix fallback from sse to longpolling to avoid infinite request loop.
- Abort the SSE request when timing it out.
- Updated tests to detect this condition.
- Updated MemoryHost to throw OperationCancelledException to mimic iis.

#1121
Made changes per review.
Updated XML docs to indicate that clients rejoin groups by default.
#1505
Distinguish between close and abort in MemoryHost.
Merge branch 'release' into dev

Conflicts:
	src/Microsoft.AspNet.SignalR.Client/Transports/AutoTransport.cs
	src/Microsoft.AspNet.SignalR.Hosting.Memory/Response.cs