Fix extras to uses constant instead of value
Enforce unit tests for @Extra
Merge pull request #733 from excilys/731_fixExtra

731 fix extra
Add a check on API and core versions
Move versions check in process() method instead of init() and use the handleError() method in case of mismatch
Improve a little bit the message notification in case of errors if there is no elements
Merge pull request #734 from excilys/313_CheckAPIVersion

313 check api version