Feature/allow nested attributes

[Pat-pGuo]

Extends work from #205

For review:

• Adds nesting to dictionary attributes and packet values
  • sub-attributes are stored as children of their parent attributes
  • sub-attributes are not visible in the global namespace, only their parents
  • vendor attributes are children of vendors, which are children of the Vendor-Specific attribute

[nicholasamorim]

I have a question about this PR. In the dictionary, @Pat-pGuo. There is this code:

self.attributes = {}
self.defer_parse = []

self.stack = AttrStack()
# the global attribute namespace is the first layer
self.stack.push(self.attributes, self.attrindex)

The code does seem to work, however, the signature for push specifies that the first argument to push is an Attribute instance and not a dict as passed. What would be the correct fix here?