Closes #53 - 'a' and 'an' is now similar, 'have.type' replaces 'be.a'.

This fixes #53. However, after making the change it seems like there is no need for the 'a'. The 'an' method is useful for foo.should.be.an.instanceOf(Object). But when can 'a' be used with the current crop of matchers?

So just see this pull request as a part of the discussion. I'm not as sure anymore whether it is a good idea at all.