Display country flag next to BitAsset

[bytemaster]

This will help people tie USD to United States Dollar and CNY to China.

[vikramrajkumar]

I think this was meant to be posted here: https://github.com/BitShares/web_wallet/issues/577#issuecomment-73135192

I think we need to use cass's icons rather than flags - flags are usually very difficult to make look natural in any design