Use new simplecov setup method from rspec-support.
Rename failure message APIs.

- failure_message_for_should => failure_message
- failure_message_for_should_not => failure_message_when_negated

Given that we are moving away from `should`,
it would be odd to keep them in these APIs.
match_for_should_not => match_when_negated.
Remove old RSpec 1 compatibility shim.

I'll be adding a new one.
Provide deprecation warnings for matchers written against the old protocol.
Label some classes as private.
Refactor duplication out of positive/negative handler.
Don't print deprecations when the matcher works on 2.x and 3.x.
Rename method `rspec_3_matcher_from` => `modern_matcher_from`.
Rephrase old doc strings.
Add deprecations for old matcher DSL methods.
Consider a legacy matcher to be a matcher.
Update changelog.