Make tracebacks more sane
Make CallError handle arguments tuple containing single string

I'm not sure but apparently nose does some exception-related magic
when tests fail and I needed that to be able to print CallError
in some cases.
Fix for Python 3
Make cosmetic fix to code structure
Merge pull request #17 from jstasiak/improve-error-reporting

Improve error reporting.
Fix #18 "@inject on an ABCMeta class does not work"