Get localized strings in Android, Selendroid and iOS

* Selendroid: extract strings from apk and add getStrings() method
* Bootstrap: remove unused getStrings() and add updateStrings() to read the strings file and update the strings used to find elements with ID location strategy.
* Android, Selendroid and iOS: add optional argument 'language' to 'session/:sessionId?/appium/app/strings' to get the strings for a specific language. Now it's a POST request.
    * If the app hasn't strings for that language, returns the default strings
    * In Android or Selendroid, if language is not setted, extracts strings from app corresponding to device language
    * In iOS, if language is not setted, extracts default strings