Add support for eprint and doi information to the references

Add support for two extra bibtex fields. These are eprint and doi.
Each of these can be specified as a field in the bibtex, and will cause
the references to contain eg: arXiv:1209.1234 and doi:10.0.0.1/XXXX.

The style of each of these is controlled by the \eprint and \doi
commands.

Because they are specified as commands, the user can over-ride them to
be hyperlinks in the preamble to their tex file. The intro to the bst
file shows how this may be done.