BLD: Add version to zipline, bump to 0.5.11.dev

zipline.__version__ is now present. Closes #94.

Moreover, git master should have a .dev version string according
to convention. Releases then get the .dev label removed.

setup.py

@@ -42,7 +42,7 @@ else:
 
 setup(
     name='zipline',
-    version='0.5.10',
+    version='0.5.11.dev',
     description='A backtester for financial algorithms.',
     author='Quantopian Inc.',
     author_email='opensource@quantopian.com',

zipline/__init__.py

@@ -5,6 +5,7 @@ Zipline
 # This is *not* a place to dump arbitrary classes/modules for convenience,
 # it is a place to expose the public interfaces.
 
+__version__ = 0.5.11.dev
 
 from . import data
 from . import finance