MAINT: Change relative library imports to use dot syntax.

Testing with a Python 3 virtualenv uncovered more relative imports
that did not explicitly use the dot syntax.

zipline/__init__.py

@@ -11,7 +11,7 @@ from . import finance
 from . import gens
 from . import utils
 
-from algorithm import TradingAlgorithm
+from . algorithm import TradingAlgorithm
 
 __all__ = [
     'data',

zipline/data/loader.py

@@ -23,8 +23,8 @@ from datetime import timedelta
 import logbook
 
 from . treasuries import get_treasury_data
-import benchmarks
-from benchmarks import get_benchmark_returns
+from . import benchmarks
+from . benchmarks import get_benchmark_returns
 
 from zipline.protocol import DailyReturn
 from zipline.utils.date_utils import tuple_to_date