Fake Pocket API before each example that needs it

This makes it clearer where the fake data is coming from, and fixes a
flaky example. The Pocket API request was only being faked once at the
start of the test run, so depending on the order the tests were run they
could interfere with each other.
Clarify name of test helper methods
Configure FakeWeb before every example

Block all requests that aren't explicitly faked, and reset all faked
requests between examples to prevent them interfering with each other.