* 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
