Made some processes atomic.

[bob1de]

• Prevent events triggered by event handlers from triggering handlers
  registered via once() again.
• Prevent handlers registered from inside event handlers from firing for
  the original event as well.

[bob1de]

Another implication of this bug is the following:

>>> l = [1,2,3]
>>> for i in l:
...   print(i)
...   del l[0]
... 
1
3

If you imagine this was the list of event handlers and handler 1 (maybe conditionally) unregisters itself, handler 2 would not be executed.

The same thing happens when you first register a handler with once() and then a second one for the same event. The second handler doesn't get executed in the first run.

>>> import observable
>>> obs = observable.Observable()
>>> obs.once("some", lambda: print("first handler"))
<function Observable.once.<locals>._once_wrapper.<locals>._wrapper at 0x7f35e1206268>
>>> obs.once("some", lambda: print("second handler"))
<function Observable.once.<locals>._once_wrapper.<locals>._wrapper at 0x7f35e0b14378>
>>> obs.trigger("some")
first handler
True
>>> obs.trigger("some")
second handler
True

[bob1de]

With the fix in place, the behaviour is as expected, because a copy of the handler list is created before executing them one by one.

>>> import observable
>>> obs = observable.Observable()
>>> obs.once("some", lambda: print("first handler"))
<function Observable.once.<locals>._once_wrapper.<locals>._wrapper at 0x7f54cb1cc268>
>>> obs.once("some", lambda: print("second handler"))
<function Observable.once.<locals>._once_wrapper.<locals>._wrapper at 0x7f54caadb378>
>>> obs.trigger("some")
first handler
second handler
True
>>> obs.trigger("some")
False

[bob1de]

Closes #9

[bob1de]

Closes #10

[timofurrer]

Looks good! Thanks 🎉