Add support for tox (http://tox.testrun.org/) and Travis CI (http://travis-ci.org/)

Sample Travis CI build for my fork: http://travis-ci.org/#!/msabramo/django-gravatar/builds/1782547

To set up Travis CI for your own repo, see http://about.travis-ci.org/docs/user/getting-started/

Sample tox output:

```
[last: 0] marca@scml-marca:~/dev/git-repos/django-gravatar$ tox
GLOB sdist-make: /Users/marca/dev/git-repos/django-gravatar/setup.py
py26 sdist-reinst: /Users/marca/dev/git-repos/django-gravatar/.tox/dist/django-gravatar2-1.0.5.zip
py26 runtests: commands[0]
/Users/marca/dev/git-repos/django-gravatar/.tox/py26/lib/python2.6/site-packages/django/conf/__init__.py:75: DeprecationWarning: The ADMIN_MEDIA_PREFIX setting has been removed; use STATIC_URL instead.
  "use STATIC_URL instead.", DeprecationWarning)
/Users/marca/dev/git-repos/django-gravatar/.tox/py26/lib/python2.6/site-packages/django/conf/__init__.py:110: DeprecationWarning: The SECRET_KEY setting must not be empty.
  warnings.warn("The SECRET_KEY setting must not be empty.", DeprecationWarning)
Creating test database for alias 'default' (':memory:')...
Creating tables ...
Creating table auth_permission
Creating table auth_group_permissions
Creating table auth_group
Creating table auth_user_user_permissions
Creating table auth_user_groups
Creating table auth_user
Creating table django_content_type
Creating table django_session
Creating table django_site
Creating table django_admin_log
Installing custom SQL ...
Installing indexes ...
Installed 0 object(s) from 0 fixture(s)
test_gravatar_url (django_gravatar.tests.TestGravatarHelperMethods) ... ok
test_has_gravatar (django_gravatar.tests.TestGravatarHelperMethods) ... ok
test_gravatar_img (django_gravatar.tests.TestGravatarTemplateTags) ... ok
test_gravatar_url (django_gravatar.tests.TestGravatarTemplateTags) ... ok
test_gravatar_user_img (django_gravatar.tests.TestGravatarTemplateTags) ... ok
test_gravatar_user_url (django_gravatar.tests.TestGravatarTemplateTags) ... ok
test_invalid_input (django_gravatar.tests.TestGravatarTemplateTags) ... ok

----------------------------------------------------------------------
Ran 7 tests in 5.685s

OK
Destroying test database for alias 'default' (':memory:')...
py27 sdist-reinst: /Users/marca/dev/git-repos/django-gravatar/.tox/dist/django-gravatar2-1.0.5.zip
py27 runtests: commands[0]
Creating test database for alias 'default' (':memory:')...
Creating tables ...
Creating table auth_permission
Creating table auth_group_permissions
Creating table auth_group
Creating table auth_user_user_permissions
Creating table auth_user_groups
Creating table auth_user
Creating table django_content_type
Creating table django_session
Creating table django_site
Creating table django_admin_log
Installing custom SQL ...
Installing indexes ...
Installed 0 object(s) from 0 fixture(s)
test_gravatar_url (django_gravatar.tests.TestGravatarHelperMethods) ... ok
test_has_gravatar (django_gravatar.tests.TestGravatarHelperMethods) ... ok
test_gravatar_img (django_gravatar.tests.TestGravatarTemplateTags) ... ok
test_gravatar_url (django_gravatar.tests.TestGravatarTemplateTags) ... ok
test_gravatar_user_img (django_gravatar.tests.TestGravatarTemplateTags) ... ok
test_gravatar_user_url (django_gravatar.tests.TestGravatarTemplateTags) ... ok
test_invalid_input (django_gravatar.tests.TestGravatarTemplateTags) ... ok

----------------------------------------------------------------------
Ran 7 tests in 3.589s

OK
Destroying test database for alias 'default' (':memory:')...
pypy sdist-reinst: /Users/marca/dev/git-repos/django-gravatar/.tox/dist/django-gravatar2-1.0.5.zip
pypy runtests: commands[0]
Creating test database for alias 'default' (':memory:')...
Creating tables ...
Creating table auth_permission
Creating table auth_group_permissions
Creating table auth_group
Creating table auth_user_user_permissions
Creating table auth_user_groups
Creating table auth_user
Creating table django_content_type
Creating table django_session
Creating table django_site
Creating table django_admin_log
Installing custom SQL ...
Installing indexes ...
Installed 0 object(s) from 0 fixture(s)
test_gravatar_url (django_gravatar.tests.TestGravatarHelperMethods) ... ok
test_has_gravatar (django_gravatar.tests.TestGravatarHelperMethods) ... ok
test_gravatar_img (django_gravatar.tests.TestGravatarTemplateTags) ... ok
test_gravatar_url (django_gravatar.tests.TestGravatarTemplateTags) ... ok
test_gravatar_user_img (django_gravatar.tests.TestGravatarTemplateTags) ... ok
test_gravatar_user_url (django_gravatar.tests.TestGravatarTemplateTags) ... ok
test_invalid_input (django_gravatar.tests.TestGravatarTemplateTags) ... ok

----------------------------------------------------------------------
Ran 7 tests in 3.392s

OK
Destroying test database for alias 'default' (':memory:')...
Traceback (most recent call last):
  File "app_main.py", line 51, in run_toplevel
  File "/usr/local/Cellar/pypy/1.9/lib-python/2.7/gzip.py", line 371, in flush
    self._check_closed()
  File "/usr/local/Cellar/pypy/1.9/lib-python/2.7/gzip.py", line 145, in _check_closed
    if self.closed:
  File "/usr/local/Cellar/pypy/1.9/lib-python/2.7/gzip.py", line 353, in closed
    return self.fileobj is None
AttributeError: 'GzipFile' object has no attribute 'fileobj'
_________________________________________________________________________ summary __________________________________________________________________________
  py26: commands succeeded
  py27: commands succeeded
  pypy: commands succeeded
  congratulations :)
```