- Add a method to obtain the key of the spreadsheet.
- Fixed a bug where it was only synchronizing the first time.
Use dataType instead of the accepts header

It's simpler this way.
Fixed a bug causing lastChapterReadName set to undefined when using the mark as read button
Merge branch 'hotfix-readManga' into develop
Merge pull request #2 from inorichi/develop

Synchronization now working