Fixes #29

* Update getSubPath function in /tasks/smushit.js to normalize
  filepath parameter similar to how the fileDir parameter is
  normalized. This fixes #29, in which src patterns such as ./img
  create an extraneous g/ directory in the dest directory.
* Update version in package.json to v1.2.1.
* Update release history in README with v.1.2.1 deets.