Add transform tests

This pull request adds some new tests that verify the output of works/{doi}/transform for a number of content types:

1. application/rdf+xml
2. text/turtle
3. text/n-triples
4. text/n3
5. text/x-bibliography
6. text/bibliography
7. text/plain
8. application/x-research-info-systems
9. application/vnd.crossref.unixref+xml
10. application/unixref+xml
11. application/vnd.crossref.unixsd+xml

The following content types are excluded from this test:

1. application/x-bibtex

I've excluded this one for now as I'm getting exceptions converting anything to x-bibtex. I think it is because of the sample input data I'm using, this issue might just go away when we decide on our full test data set.

2. application/json

I've excluded this at it is the default in use for all other tests.

3. application/citeproc+json

I've excluded this at is uses the same implementation as application/json.

4. application/vnd.citationstyles.csl+json

I've excluded this at is uses the same implementation as application/json.