adds options for client certs to downloader class

Adds :client_cert to the env array and then passes it into the curl
options array
adds param for client certs

adds an option to the command parser for client certicates
adds client cert
adds client cert to default config
adds accessor for client_cert
passes client cert option from Vagrant file to box_add
Merge branch 'master' into client-certs
Merge branch 'client-certs' of https://github.com/sthulb/vagrant into sthulb-client-certs

Conflicts:
	config/default.rb
	lib/vagrant/action/builtin/handle_box_url.rb
	plugins/commands/box/command/add.rb
kernel/v2: properly set default value for client cert
Update CHANGELOG