Implement setting an object's property dynamically with strings

An object's property can now be set by specifying a string instead of an
variable.
A not defined property of an object can now be set, using variables and
strings.

Tests for these cases are added, accessing different variables to ensure
that it works properly, if the property is not yet defined.
Merge pull request #566 from steffengy/fix-dynamic-properties

Implement setting an object's property dynamically with strings