Added install to CMake

Installs libsocketiopoco and necessary headers.
Changed CMake minimum version number

2.8.9 version is not required by the CMakeLists.txt, changed to a lower version number.
Fixed user namespace pollution

Moved all socket.io-poco to sio_poco namespace and removed all using Poco::...
Changed receive buffer size in SIOClientImpl::receive()

Changed static 1024 bytes receive buffer to a dynamic receive buffer based on WebSocket
receive buffer size.