Support negative counter increments and decrements. Ref #96
Add EUnit to test negative increments and decrements
Dialyzer specs on emcntr should reflect allowed arguments

We no allow negative integers for inc/dec args, so the specs have
been changed to reflect this.
Merge pull request #97 from basho/bug/lenary/negative-increments

Support negative counter increments and decrements

Reviewed-by: macintux,seancribbs