Merge branch 'key_store_and_accounts_integration' of https://github.com/Gustav-Simonsson/go-ethereum into Gustav-Simonsson-key_store_and_accounts_integration

* 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

Merge branch 'import_presale_keys' of https://github.com/Gustav-Simonsson/go-ethereum into Gustav-Simonsson-import_presale_keys

Conflicts:
	crypto/crypto.go

* Browser now has tabs
* Fixed a callback issue

* 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

KeyStore (Low level key functionality)

* Further simplify "constructor" function's allocation of structs
* Fix formatting

* Allocate with composite literal instead of new
* Remove check of number of bytes read from rand

Private key \x00\x00...\x00 returns the _exact_ same public key as \x11
\x11...\x11. Currently investigating.

Returning an error would indicate a complete failure initialising the
Ethereum backend. Instead we should print the message and continue.

rlp: allow encoding non-empty interface values

Difficulty was broken when refactored.

This needs to be supported because []someInterface does occur sometimes.

Funny enough, the fix involves changes to the decoder. makeDecoder
cannot return an error for non-empty interfaces anymore because the type
cache builds both decoder and writer. Do the check at 'runtime' instead.

rlp: add functions for encoding

* 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