Ability to create single purpose signals for emitting

[shaunduncan]

Currently the only way to emit signals is, surprisingly, by calling emit with the signal to send. This isn't bad, but it makes re-use a bit poor. What I should be able to do, is something like:

import smokesignal

my_signal = smokesignal.emitter('foo')

my_signal.emit()  # emits the 'foo' signal

Up in the air: should this work as a context manager?

with my_signal:
    pass