YAML support for google_json_key_string

[SaxtonDrey]

Hi.

I tryed to use google_json_key_string added by #415 .
But there is no way to load google_json_key_string from YAML file because it isn't present in Config#load_yml .

asset_sync/lib/asset_sync/config.rb

Lines 226 to 231 in 88e7a64

	self.rackspace_api_key = yml["rackspace_api_key"]
	self.google_json_key_location = yml["google_json_key_location"] if yml.has_key?("google_json_key_location")
	self.google_project = yml["google_project"] if yml.has_key?("google_project")
	self.google_storage_secret_access_key = yml["google_storage_secret_access_key"] if yml.has_key?("google_storage_secret_access_key")
	self.google_storage_access_key_id = yml["google_storage_access_key_id"] if yml.has_key?("google_storage_access_key_id")
	self.existing_remote_files = yml["existing_remote_files"] if yml.has_key?("existing_remote_files")

[SaxtonDrey]

I made PR #419