Bug 979717 - Change Evernote sync to perform delayed download of resources

1. Note resources (images) are downloaded and cached the first time a note is opened.
2. Note resources larger than max size (currently set to 2MB) are not downloaded.
3. Notes with resources larger than max size cannot be edited.
4. Notebook view (list of notes) no longer shows preview of first image in note.
5. Change code to start downloading contents as soon as first sync chunk is received.
6. Added a sync retry counter - if sync fails to complete, on restart ask user if they want to retry.
Merge pull request #15 from yor-mozilla-com/fix979717

Bug 979717 - Change Evernote sync to perform delayed download of resourc...