xhr.py and xhrstreaming.py accepts generic arguments upon connection.

Tornado allows request parameters to come from path section of the URL not only from query section (?name1=value1&name2=value2)

For example if Tornado handler url pattern is "/api/kernels/(?P<kernel_id>\w+-\w+-\w+-\w+-\w+)/shell" and real request comes for "/api/kernels/2-3-4-2-4/shell" handler will be executed with named argument kernel_id='2-3-4-2-4'.

Unfortunately sockjs-tornado didn't have this feature for the connection set up request - all parameters should be passed in a query section of URL. This change tries to fix that.