skip self/cls in the argspec

Instead of get_arg_names, the behaviour of which is non-obvious in the
presence of keyword-only arguments, get_arg_spec is introduced that
returns a modified argspec without self or cls.
add support for keyword-only arguments

Keyword-only arguments are mapped to optional arguments. They are
required iff they have no default value.
fix conflicting option detection with kwonly
remove python2.7+ syntax
add tests for keyword-only arguments
Merge pull request #53 from Tanriol/feature-kwonly

Keyword-only arguments support (Python3)