* Use crypto.Sign instead of directly calling secp256k1 lib
* Rename UserAccount to Account and Addr to Address (for consistency)
* Change AccountManager.Sign to take ptr to Account instead of
address byte array
* Simplify copying of Accounts in Accounts()
* PubkeyToAddress and GetEntropyCSPRNG now exported
* Add initial UserAccount and AccountManager structs
* Add NewAccount, Sign and Accounts functions
* Refactor key stores to use key address as main identifier
while keeping the UUID.
* Use key address as file/dir names instead of UUID
* ImportPreSaleKey takes a KeyStore, a presale key JSON (e.g. file content)
and a password string. It stores the key in the given key store.
* Refactored common AES decryption and moved some functions to crypto.go
