Enable request to local files

When a request to local files using XMLHttpRequest succeeds, it returns a status equal to zero (0) and
with responseText attribute filled. These changes make viewer.js return an ajaxSuccess() when those conditions have been meet.
Merge branch 'local-files-request' of github.com:appglu/viewer.js -- closes #47

* 'local-files-request' of github.com:appglu/viewer.js:
  Enable request to local files