[Content] Attributes 'modifiedDate' and 'releaseDate' defined in 'Content' interface should be 'Date' type

Add conversion from magic format to JS Date for given fields.

All mutators/accessor of ContentItem were adjusted to chromium convention.
Several minor fixes.

BUG=XWALK-2803
Merge pull request #391 from tiwanek/XWALK-2803

[Content] Attributes 'modifiedDate' and 'releaseDate' fix in Content API