added support for casperjs. casperjs apparently requires phantomjs be on the path.
added config options to:
allow passing environment variables
override default tmpdir location

Phantomjs.configure do |config|
  config.phantomjs_path = "#{BASE_PATH}/bin/casperjs"
  config.phantomjs_env_path = { "PATH" => "#{ BASE_PATH }/bin:#{ ENV['PATH'] }" }
  config.phantomjs_tmpdir = "#{BASE_PATH}/tmp"
end