Added support for __slots__ and saving binary data to class variable

Added support for class attributes not included in `_field_order` by defining in `__slots__`, tests and example in reaadme added.

To simplyfy attribute search in `getattr` and `setattr` the metaclass now collects slots from self and any super class that is not `structObject`

Additionally, the binary data provided during `structObject.unpack()` is now saved to `structObject._bindata`. This was added to allow `structArray.len()` to compute it's length not explicetly stated elsewhere in a data structure.
Merge branch 'master' of https://github.com/wrenoud/structObject