Add assertResponseContains & assertResponseNotContains methods

[goodtune]

A couple of convenience methods to save some typing in test cases.

def test_response_contains(self):
    self.get('hello-world')
    self.assertResponseContains('<p>Hello, World!</p>')

def test_response_not_contains(self):
    self.get('hello-world')
    self.assertResponseNotContains('<p>Hello, Frank!</p>')

Uses the private _which_response method and defaults html to True.

[coveralls]

Coverage decreased (-1.5%) to 95.565% when pulling d9a1ad3 on goodtune:assertResponseContains into b7bd744 on revsys:master.

[coveralls]

Coverage increased (+0.07%) to 97.177% when pulling 5b50303 on goodtune:assertResponseContains into b7bd744 on revsys:master.

[coveralls]

Coverage increased (+0.07%) to 97.177% when pulling ada6db8 on goodtune:assertResponseContains into b7bd744 on revsys:master.

[coveralls]

Coverage increased (+0.07%) to 97.177% when pulling 0d32908 on goodtune:assertResponseContains into b7bd744 on revsys:master.

[coveralls]

Coverage increased (+0.07%) to 97.177% when pulling 0d32908 on goodtune:assertResponseContains into b7bd744 on revsys:master.

[coveralls]

Coverage increased (+0.07%) to 97.177% when pulling 0d32908 on goodtune:assertResponseContains into b7bd744 on revsys:master.

[frankwiles]

Awesome thanks!