Refactor juju/errors to not rely on juju/errgo
Add a "NotValid" error.
Fix a typo.
Merge pull request #7 from ericsnowcurrently/not-valid

Add a "NotValid" error.
Merge branch 'master' into refactor
Update the NotValid error to work with the new world.
Make errors compatible with errgo
Update docs, and simple makefile.
Updates following review.
Merge pull request #10 from howbazaar/refactor

Refactor juju/errors to not rely on juju/errgo