Added HP3456a System Multimeter

Here is that HP3456a class I've been working on. There are a couple of warts left; the 3456a is an old instrument, that especially when set at high nplc and number of successive readings in one trigger will need adequate thinking time. The gpib_firmware settings needed to achieve this are set at init right now. These would need a similar treatment to self.terminator, but I thought it best to let you decide where that should go. After all, these settings would break with gpib_firmware<5.

Note the additional wild testfile in instruments; it is there to give you an idea of how this thing communicates.
``Enum``s are great for organizing code, but in general, an ``Enum`` that reduces to ``bool`` should be replaced by just actually using a ``bool``. Here, you have several ``Enum``s that each allow only ``on`` or ``off``, so making ``dmm.filter`` a ``bool`` using [``bool_property``](http://instrumentkit.readthedocs.org/en/latest/devguide/util_fns.html#property-factories) expresses the same intent.