Put methods from spec/spec_helper in spec/support

There were a bunch of free-floating methods in spec_helper. They were
pulled over from test/helper, but they shouldn't be there. Move them
into appropriate modules in spec/support. This commit also cleans up the
"should_accept/reject_dummy_class methods in favor of using proper
matchers directly. This commit should probably not get squashed or git
will likely forget that those matcher specs are the same file because of
how much changed.
Move fixtures to spec/support/fixtures
Remove extraneous file
Change contribution guidelines to mention RSpec
Reinstate inclusion of shoulda_macros
Missing newline in the 4.1 gemfile
No need to differentiate constants in FakeRails
Move the s3 spec's rails_env to the end
Give all spec names the correct tense