Change the way that eprint repository prefixes are processed.

The previous technique was smart, but did _not_ match the stipulations in
http://arxiv.org/help/faq/references -- these require that the specific string
arXiv:yyyy.nnnn, with a leading space, and warn not to have a '{' just before
the 'arXiv:' (who knows...).  The previous version did have a close-up '{'.

This technique is slightly clumsier in some ways, but is more straightforward in
the .bst file, and requires no LaTeX magic in the style file.

Incidentally changed the test framework in the test/ directory, to support
multiple independent tests rather than a compendium test-everything one..