Fix bug 2678 - Adding proxy settings for GoogleSpell

Implements code supplied in the bug report

http://www.tinymce.com/develop/bugtracker_view.php?id=2678

Adds support for proxies using Curl in the GoogleSpell class. I have
name spaced these under GoogleSpell.

New settings are set in config.php
GoogleSpell.proxy.type    - http or socks
GoogleSpell.proxy.server - e.g. my.proxy.server
GoogleSpell.proxy.port - e.g. 3127
GoogleSpell.proxy.auth - e.g basic
GoogleSpell.proxy.user - e.g. username
GoogleSpell.proxy.password - e.g. password
GoogleSpell.connection.timeout - e.g. 10