Address author's PR feedback

https://github.com/bendyworks/hspec-wai/commit/94b151ecb333582aa1356554a427b71db24c0827

- rename Test.Hspec.Wai.Expectation to Test.Hspec.Wai.Expectations
- add documentation to lift Test.Hspec functions
- import Test.Hspec.Expectations rather than Test.Hspec to future-proof