Another draft for a variable class in templates
Transforms the result of a filtered var into a Link_VariableInterface
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