Merge pull request #1576 from pfmoore/executable_wheel

Modify __main__.py to allow running from wheel

pip/__main__.py

 import sys
+
+# If we are running from a wheel, add the wheel to sys.path
+# This allows the usage python pip-*.whl/pip install pip-*.whl
+if __package__ == '':
+    import os
+    path = os.path.dirname(os.path.dirname(__file__))
+    sys.path.insert(0, path)
+
 import pip
 
 if __name__ == '__main__':