expressions: Fix indentation
Add Clock documentation
Make Clock.advance() stop on scheduled event

Instead of keeping remaining advancement time separately,
schedule an event at the appropriate time and then just check if
it has elapsed.
This event is special in that it is scheduled *after* other
events of the same time.

Allow advance() to be called with arbitrary futures, or with
None which will "run forever" (as long as events are still
scheduled).

Add tests for the new functionality.

Also, improve documentation.
Run doctest when testing
Document expressions
Add docs for the (yet nonexistent) signal module

Look ma, doc-driven development
Very very simple implementation of signals
signals: Implement weakref listeners
signals: Design what happens when a listener is connected multiple times
signals: Add return value collection and signal chaining
Implement per-insance signals
signals: s/receiver/listener/
signals: Add support for arg_adapter
Call class signal automatically from instances
LICENSE: We're not really based on Blinker any more

... but let's still kep the thing there
docs OCD
Make signal connection lazy
Allow naming and documenting signals
Support Signal signatures
Simplify Signal creation