added default stream name option

[fingerpich]

In most cases users don't need to change streamName for each sendEvent calls so I think they should be able to change the default stream name
Also users may confuse streamName and google analytic eventName and I think it would help users to know the main thing they send is jsonData.

myIngrow.sendEvent(jsonData) // stream name would be "events"
myIngrow.setDefaultStreamName("stream_name")
myIngrow.sendEvent(jsonData) // stream name would be "stream_name"