Add Hypothesis for property-based tests

Hypothesis:
"It works by letting you write tests that assert that
something should be true for every case, not just the ones you happen to
think of."

I think it's well suited for the task of ensuring Daphne conforms to the
ASGI specification.

.gitignore

@@ -4,3 +4,5 @@ __pycache__
 dist/
 build/
 /.tox
+.hypothesis
+.cache

.travis.yml

@@ -11,6 +11,6 @@ env:
   - TWISTED_RELEASE="twisted"
   - TWISTED_RELEASE="twisted==16.0.0"
 
-install: pip install $TWISTED_RELEASE -e .
+install: pip install $TWISTED_RELEASE -e .[tests]
 
 script: python -m unittest discover

setup.py

@@ -27,6 +27,9 @@ setup(
         'twisted>=16.0',
         'autobahn>=0.12',
     ],
+    extras_require={
+        'tests': ['hypothesis', 'tox']
+    },
     entry_points={'console_scripts': [
         'daphne = daphne.cli:CommandLineInterface.entrypoint',
     ]},

tox.ini

@@ -3,6 +3,7 @@
 envlist = py{27,34,35}-twisted-{old,new}
 
 [testenv]
+extras = tests
 deps =
     twisted-old: twisted==16.0.0
 commands = python -m unittest discover