Provide map functions from `citation` to various ontologies & standards

[cboettig]

knitcitations could benefit from a more rigorous structure for data handling.

• [ ] Data from crossref and other schemas should be captured in complete S4 representations of the schemas.
• [ ] Then coercion methods should be defined between R's citation class and these other representations
• [ ] format/print methods should be added to extend R's citation class to render in these other formats (.e.g. the RDFa formats)