Fix mixin compile-time error

Fix #47: mixin compile-time error (#48)

Additionally,
* Update examples in documentation
* Update to_string_helper and to_string_helper_generator to version 1.1.1

v1.1.0

• Added annotation @ToStringMixin.

v1.0.5

• Improved documentation.

v1.0.4

• Documented implemented features.

v1.0.3

• Added helper class ToStringHelper. This class allows to be used without code generation.
• Added annotation @ToString and @ToStringField, which allow to configure the generated code for toString() method.