Increased Flexibility

This release deprecates the `@has_property()` decorator in favour of a new,
more flexible `@wrapper_property()` decorator. With this decorator it is
possible to do **anything** that is otherwise possible via the standard
`@property`-way of creating properties. It provides convenient shortcuts to
create commonly-needed properties.

Furthermore, this release improves the `@with_init_from_properties()` and
`@with_repr_like_init_from_properties` decorators so that they only use read-
and settable properties. Before, also properties without setter were shown in
`__repr__`, which caused an `AttributeError` to be raised in `__init__`.

This release fixes two bugs

- #3: the default value in `@has_property` was exactly the same object
  for all instances which caused problems with mutable types like
  `list`s.
- #4: __repr__ indentation was not pretty for nested arguments

This is the first release providing basic functionality:

- quickly adding properties to classes via decorators
- adding an __init__-method based on the properties
- adding a pretty __repr__-method based on the properties and the
  __init__ method
- adding an __eq__-method which compares the properties