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

* 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

I'm reasonably confident that the encoding matches the output of
ethutil.Encode for values that it supports. Some of the tests have been
adpated from the Ethereum testing repository.

There are still TODOs in the code.