Task/533/bittrex ios and exchange logos

Is this pull request adding a feature or fixing a bug?
feature

Does this pull request close an issue? If so, which one? Add Bittrex support to iOS #533

What does this pull request do? What does it change?

Adds Bittrex support to iOS (tested using same credentials I used to test macOS)
Switched to using the new PNG logos
Removed BalanceVectorGraphics dependency and moved any remaining files into the project
Forgot to push Bittrex BCC to BCH fix in my previous PR so I cherry-picked the commit into this one

balance-io / balance-open