Get rid of scikit-learn dependencies

[kmike]

Without scikit-learn and seqlearn it will be possible to install webstruct without numpy/scipy stack and without Cython. They are used for auxilary things: nicer __repr__s, Pipeline instead of hand-written classes, metrics (which are broken anyways, see https://github.com/scrapinghub/webstruct/issues/14) - that's nothing serious or hard to replace.

[tpeng]

seqlearn dependency removed by https://github.com/scrapinghub/webstruct/pull/23