* Move random entropy functions to new package randentropy
* Add function to get n bytes entropy where up to first 32
bytes are mixed with OS entropy sources
* 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
* Remove flags field from key struct
* Change JSON struct fields from string to []byte
* Change GenerateNewKey API to take io.Reader for random source
* Remove mixing entropy source function
* Use testing Fatal in tests
* Simplify scrypt constants with const block
* Add key store constructors and make their types private
* Simplify key store and file namings to be less Java Enterprise™
* Change test error logging to use t.Error(err)
* Reduce number of naked returns (just like my ex-gf)
* Simplify file reading path code
* Add new generic key_store interface
* Add new plaintext key store storing unprotected keys on disk
* Add new encrypted key store storing encrypted keys on disk
* Add new entropy mixing function using OS and go runtime sources
