Broaden the notion of `args_spec()`
Fix function spec
Add argument capture capability (fixes #86)
Fix `args_matcher.args_pattern` type spec
Change `meck:capture` return value

It returns just value (not wrapped in {ok, ...} and fails with
error:not_found if the expected call has not happened.
Merge pull request #97 from horkhe/feature/capture

Feature/capture