Switch to new expect syntax.

This was a bit more difficult than for rspec-core and
rspec-mocks, since we still need to test the old
`should` syntax. I've defined a shared_context, available
by tagging a group with `:uses_should`, that makes `should`
temporarily available to examples in that group.
Remove unnecessary spec_helper requires.

These cuke scenarios don't create a spec_helper file.
Instead, the `spec/spec_helper` file (used by rspec-expectations'
specs, but not intended for use by the cukes) was being loaded.
It was causing failures because it disables `should` but I hadn't
gone through the cukes yet.
Merge pull request #196 from rspec/switch_to_new_expect_syntax

Switch to new expect syntax.