Simpler regex for email matching.

Previous regex has some pathological cases (@chekunkov observed it taking 2h+ CPU on a single page).
Also, for extraction task we are more interested in real-world email addresses, not in following RFCs
add dev requirements - they are needed only to run tests and build docs
add functions and classes from webstruct.model to top-level namespace
rename htmltoken_lists argument to html_token_lists
make marisa_trie import optional
a lot of documentation improvements
requirements fixes
allow to build docs without installing lxml and scikit-learn
requirements-doc.txt
DOC more docs
rename some attributes_ to attributes
DOC better tutorial (work in progress)
DOC tutorial improvements
DOC minor tutorial fixes
DOC split api.rst into several files and other doc improvements
(backwards-incompatible) move create_wapiti_pipeline to webstruct.wapiti
DOC tutorial improvements
a hook for customizing NER.extract results
NER.extarct_groups method
DOC minor tutorial improvements