Improves search and dataset list

o Improves the functions

bef.tematres.search.keyword(keyword = "tree")
bef.portal.get.dataset_list(keyword = "tree")

o Also introduces a new helper that queries a media wiki for definitions
  of terms. This is working only with the normal wikipedia media wiki
  and hast not been tested with a semantic media wiki. at the moment

bef.define_a(term="tree")