Add cache mechanism to sherpa tts

[mah92]

#1732

Delay is a major drawback of sherpa tts on android phones, which makes it unusable for average blind people using average phones. By caching the most frequent tts requests, sherpa is made as fast as lightning on most of the time, using much less cpu and battery. Some blind individials have tested this upgrade and have been happy :)
Please review and merge my pull request.