Add make target for setting up test environment

This commit replaces the 3 lines from the contributing instructions with
a single make target.  Only a tiny thing, but it's reduces the distance
from zero to tests passing, which can only be a good thing in my book.

Makefile

@@ -1,3 +1,8 @@
+testenv:
+	pip install -e .
+	pip install -r requirements/tests.txt
+	pip install Django
+
 test:
 	flake8 compressor --ignore=E501,E128
 	coverage run --branch --source=compressor `which django-admin.py` test --settings=compressor.test_settings compressor

docs/contributing.txt

@@ -116,9 +116,7 @@ requirements **in the virtualenv**::
 
     $ virtualenv compressor_test
     $ source compressor_test/bin/activate
-    (compressor_test) $ pip install -e .
-    (compressor_test) $ pip install -r requirements/tests.txt
-    (compressor_test) $ pip install Django
+    (compressor_test) $ make testenv
 
 Then run ``make test`` to run the tests. Please note that this only tests
 django_compressor in the Python version you've created the virtualenv with