Version 0.5.1 released

This version adds the true and false constants, and allow to give a name to constants. Some new characters are allowed for String literal values: ":","(",")","!","?" .

It also fix the following bugs: Make sure that Variables instances are not removed when parsing expressions, if these Variables are already existing, the Boolean result type for functions was not handled
Some new characters are allowed for String literal values: ":","(",")","!","?"