Add capability check for gerrit stream connection

Capability for gerrit stream connection is tested as follows:

* NG: End of stream has been reached without any data
* OK: data is arrived
* OK: End of stream is not reached until timeout

This patch adds capability test to doTestConnection() according to the
above policy.
Open a constant parameter "CMD_STREAM_EVENTS" to the public
Use constant parameter for gerrit stream
Merge pull request #95 from rinrinne/test-stream

Add capability check for gerrit stream connection