Add start listening to plugin

[mokarian]

• In the Android plugin code, within the CognitiveServices.java file, rename the SpeakText method to startSpeaking.
• In the iOS plugin code, within the CDVCognitiveServices.h and CDVCognitiveServices.m files, rename the SpeakText method to startSpeaking.
• In the plugin interface code, within the cognitiveservices.js file, rename the SpeakText method to startSpeaking.
• In the Native code, all files under the cognitiveservices folder, rename the SpeakText method to startSpeaking.
• In the Example app, add the startSpeaking functionality.
• Update the Plugin README description and examples with the startSpeaking method.