Error occured when push 'c' key with selected spans and without selected entities.
Refactoring
1. Use _.uniq instead of self implementations.
2. Use _.flatten instead of textAeUtil.flatten.
3. Extract a function model.annotationData.span.getEntities.
Merge pull request #75 from pubannotation/bugfix_error_at_push_c_without_enity

Bugfix error at push ’c’ without entity with span