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.

Builds on an idea originally by Antony Lewis antony@cosmocoffee.info
Make eprint and doi hyperlinks by default.

This works even if there is no hyperref package, because we define a
default \href in the bibtex file.
Merge pull request #1 from sbird/master

Add support for arxiv and doi entries

Currently assumes arXiv is the only eprint service. This has problems with ASCL entries and also entries from ADS which explicitly prefix arXiv.