generate ethereum private key on app load

[ryzhak]

1. Get API key from https://infura.io/. In the future app will get infura API keys from the server.
2. Generate ethereum private key using web3 library, for example :https://github.com/BANKEX/web3swift
3. Retrieve public address from private key
4. Save private key and public address locally. NOTICE: follow best security practices because if user would lose his private key he would also lose all his ether.