Update axiom to use axiom-types

Signed-off-by: Dan Kubb <dan.kubb@gmail.com>
Add doc explaining Axiom::Function::Binary#type
Change travis to not run mutant

* This is a temporary change until mutant allows me to exclude
  the Axiom::Types namespace from the classes it mutation tests.
Merge pull request #36 from dkubb/axiom-types-refactor

Refactor attributes to use axiom-types