Add capability to get nested items using dot

[mike24zl]

Added a convenience capability to read with dot notation for nested dicts and lists in model.
Example:

>>> d = {'level1':{'level2':[{'leve3':'a'},{'level3':'b'}]}}
>>> f = warlock.model_factory({})  
>>> m = f(d)
>>> m.level1.level2[0].level3
'a'

Fixes #50

[aryahezarkhani]

Can you also add a couple of simple tests to safeguard against breaking changes
https://github.com/bcwaldon/warlock/blob/master/tests/test_core.py