make ticker parser more flexible

with this change it's possible to parse more complex json outputs (example: bitpay)

Signed-off-by: Jonas Schnelli <jonas.schnelli@include7.ch>
make tickers list sorted in preference

Signed-off-by: Jonas Schnelli <jonas.schnelli@include7.ch>
added some tickers (bitcoinaverage, bitpay)

Signed-off-by: Jonas Schnelli <jonas.schnelli@include7.ch>