Introduces bef.options(tematres_url="www.bar.de")

The URL of the tematres server is now settable via the bef.options()
This is discussed in and closes #11. The following examples show how
it works:

bef.options(url="befdatadevelepment.biow.uni-leipzig.de")
bef.options("url")
bef.options(url="www.fu.de", tematres_url="www.bar.de")
bef.options("tematres_url")
bef.options(tematres_url="www.baz.de")
bef.options()