Fix packaging

Fixed a mistake in setup.py. Previous release was borked.

Version 0.5.1

Instead of a single decorator @event for both normal functions and bound methods, we
now have separate decorators for each case:
observable_function for functions, and
observable_method for methods.
There are two reasons for this. First, the tom-foolery I had to pull to get the single decorator
implementation working was fragile and not extensible (for example, we may wish to support
making observable class and static methods in the future). Second, by switching to two separate
decorators now we keep unification into a single decorator in future releases an available option,
and so lose nothing in terms of future possibilities.

For developers, another important aspect of this release is that the code should now be a lot
easier to understand because every task is handled by exactly one specific class.

Many missing doc strings have been added.