Add no-exclusive-tests rule, closes item:644

Handle loading manually until ESLint has proper custom rule loading via npm
modules.
Merge test ESLint config

… until ESLint gets config extends support, see:
https://github.com/eslint/eslint/issues/963
Merge pull request #438 from tlvince/refactor/eslint-ddescribe

Add no-exclusive-tests rule, closes item:644