DOC: fix import and support python3 for beginner tutorial

The beginner tutorial Dual Moving Average example attempted to import
extract_transactions from the wrong location.

The tutorial and corresponding example also would fail using python3
due to indexing the view object returned via context.exchange.values()

catalyst/examples/dual_moving_average.py

@@ -84,7 +84,8 @@ def handle_data(context, data):
 def analyze(context, perf):
 
     # Get the base_currency that was passed as a parameter to the simulation
-    base_currency = context.exchanges.values()[0].base_currency.upper()
+    exchange = list(context.exchanges.values())[0]
+    base_currency = exchange.base_currency.upper()
 
     # First chart: Plot portfolio value using base_currency
     ax1 = plt.subplot(411)