Changes how the variables are transformed into Link_Variable

Now, the transformation takes place as soon as setValue() is called,
instead of getting it from offsetGet
Add the [] case on offsetSet in Link_Variable
Remove Environment dependency on Variable class
Refactor a getter, fix a bug on the filters
Alter getter & setter for the Variables
Alter travis.yml -- remove coverage, add php 5.5
Add the possibility to iterate on simple objects in Variable
Greatly improve Variable behaviour on objects
Add some tests for the Variable class
Add a magic method check on __set & __get magic methods
Alter __call method in Variable
Checks that the filter is a Variable, and act accordingly
Make useful a verification on filter method
Add two methods in the VariableInterface
Add a new interface to handle extensions
Move around an item in the CHANGELOG
Add a built-in extension
Change how filters are loaded
Converts and filters Filters into the Extension format

As the filters are easier to edit, a huge amount of them are now gone from
the original package. They will, maybe, be back in another built-in
extension.

Also, enhance the protect filter.
Update CHANGELOG