Add rspec matcher

[tmaier]

It would be cool to have an rspec matcher included with the gem.

This is how an older gem is solving this: https://github.com/validates-email-format-of/validates_email_format_of/blob/master/lib/validates_email_format_of/rspec_matcher.rb

[karlwilbur]

I like this idea. I'll look into it.