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)