BibTeX citeproc updates: missing fields and 'proceedings' and 'misc' types

Updated handling of BibTeX to CSL conversion:

- `@proceedings` type should be mapped to `book`
- mapping `school`, `institution` and `organization` fields to `publisher`
- handle `@misc` type as "no type", rather than article. Also map `howpublished` field to `publisher`

I have not added any tests, since I don't know Ruby well. Also, a couple tests were failing on master before these changes anyway..

Addresses issues #72 and #85.