Add GreenAddress instant verify plugin

GreenAddress wallet provides "instantly confirmed" transactions based on server-side-signed multisignature transactions, with the server verifying that outputs were not spent before. 

This plugin allows checking if some given transaction with output addressed to an Electrum wallet was sent using the GreenAddress instant confirmation feature. It should work with 'standard', 'trezor', and 'btchip' wallets.

It works similarly to https://greenaddress.it/en/instant/ which allows checking if given txhash is sent using instant confirmation. To avoid privacy issues with exposing instant transactions publicly, GreenAddress requires a signature from a key matching some output of the given transaction. It is verified by a simple GET request to https://greenaddress.it/verify/?signature=%s&txhash=%s (line 114)

As a side note, we also have a [BIP-70 enhancement proposal](https://github.com/greenaddress/bips/blob/bip-payment-request-instant-confirmations/bip-payment-request-instant-confirmations.mediawiki) pending review which allows verifying instant confirmations via the payment protocol.