Proxy support

[pete-otaqui]

Hi there,

I've added support for the http_proxy / HTTP_PROXY environment variable. Currently only tested on my mac.

I think that ideally using the environment variable or not should be an option to the constructor. Maybe something like:

@browser = Selenium::Client::Driver.new( :http_proxy => true ) # use environment variable
@browser = Selenium::Client::Driver.new( :http_proxy => 'http://user:pass@proxy:port' ) # use this proxy
@browser = Selenium::Client::Driver.new( :http_proxy => false ) # no proxy (the default)