Add assert_raises assertion
Add build status to readme
Split up assertions from the rest of the tests
Rename `assertion` to `assertion_error`
Prefer curly-braces to do/end inside methods

That goes for other blocks aswell
Extract test prefix to constant
Quell ruby-warnings
Clarify test
Refactor Test.runnable methods

- Move method order depending on test_order to a private method
Change spec method prefix from "test_" to "spec_"

Every subclass of Runnable should be able to know how to run it's own
tests.
Move .runnable_methods and PREFIX to Spec class
Rename DSL#create to DSL#create_sub_klass
Refactor: extract regexp to variable