allow pass different subsequence method in similar_region
allow override the build_extraction_tree in InstanceBasedLearningExtractor
fix base on the comments

* fix pep257 warning
* use max instead if nlargest
try annotate next fragment in template maker when best_match is false

sometime the data to extract are same (e.g. the rating is restaurant reviews), instead of raise exception, this change make the template maker try to annotate next match ones.
Revert "try annotate next fragment in template maker when best_match is false"

This reverts commit 32e8dc852e5cdde4dfe8535525ce48f20d8eae12.
promote the _attrs2dict and _labelled to public
let annotate_fragment return an boolean
Merge pull request #47 from tpeng/patch-1

allow passsing different subsequence methods to similar_region