Make tests work

Fix a number of problems that I had while trying to run tests via `tox`, including but not limited to:

```
I can't run the tests -- by any chance did you forget to check in a `conftest.py` file or something?

```
❯ tox -e py27
GLOB sdist-make: /Users/marca/dev/git-repos/pytest-httpbin/setup.py
py27 inst-nodeps: /Users/marca/dev/git-repos/pytest-httpbin/.tox/dist/pytest-httpbin-0.0.3.zip
py27 runtests: PYTHONHASHSEED='3043088091'
py27 runtests: commands[0] | ./runtests.sh
============================================================================= test session starts ==============================================================================
platform darwin -- Python 2.7.6 -- py-1.4.26 -- pytest-2.6.4
collected 12 items

tests/test_httpbin.py EEEEEEEEE
tests/test_server.py EEE

==================================================================================== ERRORS ====================================================================================
_________________________________________________________________ ERROR at setup of test_httpbin_gets_injected _________________________________________________________________
file /Users/marca/dev/git-repos/pytest-httpbin/tests/test_httpbin.py, line 6
  def test_httpbin_gets_injected(httpbin):
        fixture 'httpbin' not found
        available fixtures: capsys, capfd, recwarn, pytestconfig, monkeypatch, tmpdir
        use 'py.test --fixtures [testpath]' for help on them.

/Users/marca/dev/git-repos/pytest-httpbin/tests/test_httpbin.py:6
```