Some improvements in BTE configuration

Hi Keiji,

your PR (https://github.com/DSpace/DSpace/pull/386) seems very OK. I am glad you tried out BTE and managed to write your own modifier as well.

I send you this PR (over your PR) since I have made the following changes:

1) I have changed the names of the keys of the three services (pubmed, crossref and arxiv) so as to be the same as the element names in the corresponding XMLs

2) I cleared some configuration that is not needed + commented out configuration that cannot be used by default

3) Minor code fixes

Thanks,
Kostas