Make the landmark manager an attr dict

[patricksnape]

This is a special kind of mutable mapping that exposes it's
keys as attributes. Uses the correct overriding of
__getattr__ and __setattr__ in order to not override the default
behaviour of the attributes actually on the class (inspired
by Bunch and used under the MIT license:
https://github.com/dsc/bunch/blob/master/LICENSE.txt).

This also makes a stronger enforcement of ASCII style string
identifiers only for Python 2 and Unicode identifiers for
Python 3.

An example usage is:

import menpo.io as mio

lena = mio.import_builtin_asset.lenna_png()
# Access the landmarks
lena.landmarks.LJSON
# And getitem style access style works
lena.landmarks['LJSON']
# And setting works sensibly too
lena.landmarks.new_key = lena.landmarks.LJSON
lena.landmarks['another_key'] = lena.landmarks.LJSON
# And blocks bad keys
lena.landmarks['3'] = lena.landmarks.LJSON  # Fails due to invalid Python identifier

@jabooth @nontas @grigorisg9gr What do you think? Obviously requires menpo#675 first

[patricksnape]

@jabooth Hilariously this fails as a key because def is a reserved keyword

[jabooth]

Interesting! So this seems to match exactly what we would expect from duck typing onto a class...

[jabooth]

@patricksnape (I'll be honest, before trying that I actually thought the first two might succeed, as I'd seen the latter example work before. But, (obviously, in retrospect) __builtin__ != keyword)