Alias method for Rspec protocol compat.

Hi @r7kamura

A follow up to #3. Adds method name from Rspec 3 protocol as an alias just to prevent noisy deprecation statements.

Notes:
- Pre-Rspec 1.2 failure message protocol was `failure_message` and `negative_failure_message`
- Rspec 2 protocol was `failure_message_for_should` and `failure_message_for_should_not`
- Rspec 3 protocol is `failure_message` and `failure_message_when_negated`