Also fix misc bugs
* Update README.md doc.
* Fix relationships not shown in JSON API for a given paper if relationship was
not referenced.
* Handle tags
* Update README.md documentation.
* Fix "cite" relationship import. Papers created when parsing references
were not given a "cite" relationship.
TODO: The latest fix creates too much recursion when adding a given
paper. Adding a paper now basically means "crawling arXiv".