rephrase introduction

Tied models and learners to the scikit-learn concept of an "estimator".
Also renamed "SVMs" to "learning algorithms".
starting
add constraint pruning to NSlackSSVM.
warm start fixes in n_slack_ssvm
some cleanups and warm start fixes in n_slack_ssvm
Merge branch 'master' of github.com:pystruct/pystruct
Merge branch 'pruning_n_slack'
Making the structure of X and Y clearer for GraphCRF closes # 26
set default infernce algorithm for CRFs in base crf class
discover "best" installed inference algorithm for default. Speeds up tests a lot.
don't use ogm as default as it breaks to many tests. set the inference method hard to lp in fragile / exact tests.