Export is fully implemented

- Export list of feeds (OPML)
- Export list of favourites (JSON)
- Export list of articles per feed (JSON)
Export form is not displayed if there is nothing to export
Improve import / export functionnality

It is not finished yet and does not even work at all!!
- ZIP archive can be uploaded
- Entries are imported from starred.json and feed*.json but not added in DB for
  the moment.
- Fix export (add author, id -> guid, content -> content.content and add
  alternate)

See https://github.com/marienfressinaud/FreshRSS/issues/163
Fix typo + improve detection OPML file
Better OPML import / export

- use a new OPML library (https://github.com/marienfressinaud/lib_opml)
- import has been completely rewritten (far better!)
- introduce addFeedObject and addCategoryObject (in DAO for the moment).
  Permit to add easily feeds and categories (check if they already exist in DB)
- introduce html_chars_utf8 (wrap htmlspecialchars for UTF-8)
Import of articles is implemented!

- Remove massiveImportAction and addCategories from FeedController
- Fix typo for some methods (camelCase)
- addCategoryObject and addFeedObject return id if corresponding object already
  exists in DB
- introduce addEntryObject. Return -1 if Entry already exist (in order to keep
  quite good performances)
- Complete importArticles method

Need some more tests + better performance
Improve OPML rendering

- Fix html entities rendering
Fix coding style
Merge branch '163-export' into dev

Conflicts:
	app/layout/aside_feed.phtml
Merge branch 'dev' of github.com:marienfressinaud/FreshRSS into dev