🐛 Fix `pyquid.Switcher.get_quid` for Quiddity initialization

see: #17 (closed)

This merge request adds a quiddities read-only list member attribute on pyquid.Switcher.

Whenever a pyQuiddity object is initialized (either from pyquid.Switcher.create or from the pyquid.Quiddity class) it is appended to the related pyquid.Switcher instance within its quiddities attribute.

Calls to pyquid.Switcher.get_quid(id) now looks in this list and returns a new reference to the pyQuiddity object matching the id argument for further processing.

>>> from pyquid import Switcher
>>> sw = Switcher('quidz')
>>> sw.create('dummy', 'q1')
<Quiddity (1) at 0x7fbf386529f0>
>>> sw.create('dummy', 'q2')
<Quiddity (2) at 0x7f2965f0bf38>
>>> sw.quiddities
[<Quiddity (1) at 0x7fbf386529f0>, <Quiddity (2) at 0x7f2965f0bf38>]
>>> quid = sw.get_quid(1)
>>> quid
<Quiddity (1) at 0x7fbf386529f0>
>>> quid.id()
1
>>> sw.quiddities[0] == quid
True