Add AQI support for several Chinese cities

Read tweet from US embassy's twitter account and display the AQI info
with the weather widget.

Currently, only following cities supported:
1, Shanghai: https://twitter.com/CGShanghaiAir
2, Beijing: https://twitter.com/BeijingAir
3, Chengdu: https://twitter.com/CGChengduAir
4, Guangzhou: https://twitter.com/Guangzhou_Air

Change-Id: I0fd6efbac35eb1819417c6dc8ac05b9a6e4d9949