Use form_tag instead of form_for in Sign in form
Decouple the tests from implementation

Following the example in Section 8.3.3, go through the user and
authentication request specs (i.e., the files currently in the
spec/requests directory) and add methods to spec/support/utilities.rb to
decouple the tests from the implementation. Extra credit: Organize the
support code into separate files and modules, and get everything to work
by including the modules properly in the spec helper file.
Merge branch 'exercise-8'