Added Kraken library and basic dirver. Also implemented DriverInterface interface.

	new file:   src/BitcoinExchange/DriverInterface.php
	modified:   src/BitcoinExchange/Drivers/BTCe.php
	modified:   src/BitcoinExchange/Drivers/Bitstamp.php
	new file:   src/BitcoinExchange/Drivers/Kraken.php
	deleted:    src/BitcoinExchange/Exchanges/BTCe.php
	new file:   src/BitcoinExchange/Exchanges/KrakenAPI.php
	modified:   src/BitcoinExchange/Factory.php
	renamed:    test/instanciate.php -> test/balance-bitstamp.php
	new file:   test/balance-btce.php
	new file:   test/ticker-bitstamp.php
	new file:   test/ticker-btce.php
	new file:   test/ticker-kraken.php