Add a new method app_strings(language) to get the strings for a specific language

The Appium command 'session/:sessionId?/appium/app/strings' has changed: https://github.com/appium/appium/commit/af89c299473617b7980770102f07a1205c25627b

Now it's a post command and allows an optional parameter _language_ to get strings for a specific language.