PERF: Remove module-scope calendar creations.

Remove module scope invocations of `get_calendar('NYSE')`, which cuts
zipline import time in half on my machine. This make the zipline CLI
noticeably more responsive, and it reduces memory consumed at import
time from 130MB to 90MB.

Before:

$ time python -c 'import zipline'

real    0m1.262s
user    0m1.128s
sys     0m0.120s

After:

$ time python -c 'import zipline'

real    0m0.676s
user    0m0.536s
sys     0m0.132s

tests/data/bundles/test_core.py

@@ -155,7 +155,7 @@ class BundleCoreTestCase(WithInstanceTmpDir,
 
         @self.register(
             'bundle',
-            calendar=calendar,
+            calendar_name='NYSE',
             start_session=self.START_DATE,
             end_session=self.END_DATE,
         )
@@ -369,7 +369,7 @@ class BundleCoreTestCase(WithInstanceTmpDir,
         """
         if not self.bundles:
             @self.register('bundle',
-                           calendar=get_calendar('NYSE'),
+                           calendar_name=('NYSE'),
                            start_session=pd.Timestamp('2014', tz='UTC'),
                            end_session=pd.Timestamp('2014', tz='UTC'))
             def _(environ,

tests/data/bundles/test_quandl.py

@@ -5,6 +5,7 @@ import pandas as pd
 from toolz import merge
 import toolz.curried.operator as op
 
+from zipline import get_calendar
 from zipline.data.bundles import ingest, load, bundles
 from zipline.data.bundles.quandl import (
     format_wiki_url,
@@ -28,9 +29,9 @@ class QuandlBundleTestCase(ZiplineTestCase):
     asset_start = pd.Timestamp('2014-01', tz='utc')
     asset_end = pd.Timestamp('2015-01', tz='utc')
     bundle = bundles['quandl']
-    calendar = bundle.calendar
-    start_date = bundle.start_session
-    end_date = bundle.end_session
+    calendar = get_calendar(bundle.calendar_name)
+    start_date = calendar.first_session
+    end_date = calendar.last_session
     api_key = 'ayylmao'
     columns = 'open', 'high', 'low', 'close', 'volume'
 

tests/data/bundles/test_yahoo.py

@@ -157,7 +157,7 @@ class YahooBundleTestCase(WithResponses, ZiplineTestCase):
         self.register(
             'bundle',
             yahoo_equities(self.symbols),
-            calendar=self.calendar,
+            calendar_name='NYSE',
             start_session=self.asset_start,
             end_session=self.asset_end,
         )