Added MethodNotAllowed

doc: Fixed small error in documentation for register_error_handlers

chore: Added missing .errors folder

Trying to make sphinx flask intermapping work

chore: Applied suggestions from code review

Small fixes post merge

Chore: Version update

Core: Flask-Utils is now an extension

Utils: Added is_it_true utility function

Also added documentation and tests to go along with it

Chore: Version update