Re-implement feature detection using getFeature() (bug 1097424, bug 1097429)

Caveats:
- Since getFeature() is a privileged API, this is only used in Firefox OS,
  and only from the iframed app.
- For now, only a very limited subset of features are detected, the rest
  are always set to true.
Merge pull request #794 from diox/getfeature-buchets-in-iframed-app

Re-implement feature detection using getFeature() (bug 1097424, bug 1097429)