Allow extra data in client constructor.

This PR allows a user to pass 'extra' data to the `Raven_Client` constructor that will be recorded with every event.

My use case for this is events from command-line PHP scripts, which share an error handling system with web-facing code. The command-line events currently do not have any environment information recorded with them. This PR allows me to conditionally add the `$_SERVER` variable to the 'extra' information if the PHP SAPI is 'cli'.