Add support for mnemonic input.

[MicahZoltu]

Users often don't know their private keys, only their mnemonic. They can acquire the private keys for individual accounts from MetaMask, but it would be simpler if we could just support mnemonics.

Something to consider, is it common for users to have individual keys compromised but not the mnemonic? If so then asking for a mnemonic puts more of the user's account at risk. On the other hand, users probably are pasting the mnemonic into the page as a first attempt anyway, so we aren't protecting anything by not using it. Maybe we just ask for private key and then if they happen to enter ea mnemonic we scan common derivation paths for the address we are looking for?