Support "this" (and more) as the name of the "self" argument

[nikololiahim]

This pull request allows the definition of methods and method calls in the following way:

1. with self (the previous one)

   [self] > f
   self.g self > @

2. with this:

   [this] > f
   this.g this > @

3. with any other name:

   [slf] > f
   slf.g slf > @