Added gracefully shutdown function to selenium server

I have filled the CLA.

Description
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Implements the necessary functions to let selenium server have a way to  be gracefully shutdown.

Reason
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After discuss on IRC channel, I think there is not a gracefully way to kill selenium server, I mean, to shutdown selenium server after we handles current sessions. This feature is important to make sure that tests are not broken when someones wants to kill the selenium server.

Benefits
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Let selenium server have a way to shutdown gracefully.

Drawbacks
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...

Implementation
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The actual implementation of selenium server have this command to call: 
http://SERVER:5555/selenium-server/driver/?cmd=shutDownSeleniumServer
But this call just call system.exit.

My implementation that can be called in similar way: 
http://SERVER:5555/selenium-server/driver/?cmd=gracefullyShutDownSeleniumServer
waits for node webdriverSessions.getSessions().size() to be equal to zero before call system.exit