Encode titles IN JSON only (test)
Wrap things in a function
More general form
Fixed #44 for all types of bib input

**Summary of the changes**

- new function `make_title_ascii`, responsible for replacing non-ascii
characters by their closest match
- called by all `get_***_suggestions` functions on the title
- new requirement: `unicodedata`

Tested with different inputs (`json`, `bib`) and it works.
Merge pull request #45 from tpoisot/bib-unicode-fix

Allow unicode characters in references title