Add have_method matcher

This pull request creates a new matcher (have_method) based on Rails::Generators::Testing::Assertions.
I don't like the idea of regex-based introspection but I couldn't find any clean way to run .rb file in sandbox and work with a class from there (I will try to work on that later).

This matcher can be transformed into [composable matcher](http://myronmars.to/n/dev-blog/2014/01/new-in-rspec-3-composable-matchers) at later time.

(Travis build fails at the moment because this pull request is not based on #31 but on current master)