Rename SelfDescriber to Describer
A way for Describer implementations to decine description.
Move to `stable` Minimum Stability

And add `sebastian/diff` as a dependency.
Introduce the 'diff' Utility Function
Make PhptFormat a Describer
Introduce the TypeTrait
use TypeTrait in IsType
Make IsEqual a Describer
Use Describer in Assert::assertThat

If a matcher can describe a mismatch itself, add that information to the
`AssertionFailed` message.
Merge pull request #3 from chrisguitarguy/describing

Describing Matchers