Provide function heads where needed

Elixir master (v0.14.3-dev) requires that functions with multiple
clauses and defaults supply those defaults via a function head. This
addresses this new requirement.
Merge pull request #2 from jarednorman/elixir-v0.14.3-dev-support

Provide function heads where needed
bump 0.3.2
Add checksum calculation for isbn10
Refactor isbn10 calculation
Add isbn10 and isbn13 checksum realisation
Update code.ex and code_test.ex. Replace Enum.reverse to String.reverse.