Bug yahoo finance on querying 1 instrument

Querying a single instrument input as a string "VEU" would fail due to HTTP error not found - as build_feed will iterate over "V", "E", "U"

REPRODUCE:
from pyalgotrade.tools import yahoofinance
feed = yahoofinance.build_feed("VEU", 2008, 2014, "/home/eip/Documents/tests/pyalgotrade/DailyStockPrices")

CORRECTION:
I've simply checked isinstance(instruments, basestring) and turned it into instruments = [instruments] so that the code remains unchanged