aiohttp-session is not required (#107)

* aiohttp_session is not required

* fix code coverage

* has_aiohttp_session -> HAS_AIOHTTP_SESSION

Author: oleksandr-kuzmenko

aiohttp_security/session_identity.py

@@ -6,7 +6,11 @@
 
 import asyncio
 
-from aiohttp_session import get_session
+try:
+    from aiohttp_session import get_session
+    HAS_AIOHTTP_SESSION = True
+except ImportError:  # pragma: no cover
+    HAS_AIOHTTP_SESSION = False
 
 from .abc import AbstractIdentityPolicy
 
@@ -16,6 +20,10 @@ class SessionIdentityPolicy(AbstractIdentityPolicy):
     def __init__(self, session_key='AIOHTTP_SECURITY'):
         self._session_key = session_key
 
+        if not HAS_AIOHTTP_SESSION:  # pragma: no cover
+            raise ImportError(
+                'SessionIdentityPolicy requires `aiohttp_session`')
+
     @asyncio.coroutine
     def identify(self, request):
         session = yield from get_session(request)