[Download] Fix and complete Download API

Refactored Filesytem and Download API to share code
related to virtual roots.
Pass TCT tests.
Fix example.

BUG=XWALK-1070
Merge pull request #352 from eurogiciel-oss/XWALK-1070

[Download] Fix and complete Download API
Bump version to 0.90
Merge branch 'master' of github.com:otcshare/tizen-extensions-crosswalk