Dialyzer and deprecation fixes

[flaviogrossi]

This should fix the following deprecations and dialyzer warnings:

• some undefined types in the specs
• to_char_list/1 and String.strip/1 deprecations
• char_list() deprecated type
• the warning about unused variables in the generated code (variable "string_values" is unused, etc.)

The deprecated functions requires elixir 1.3, which should be fine i guess. It that's ok, the elixir version in mix.exs and the travis file should be updated too.