Refactor __dunder__ string assignments to metadata attribute

[mvanveen]

>> d = docs.get(docs)
>> d.metadata
{
  'author': 'Michael Van Veen',
  'date': '12-10-10'
  'copyright': 'Michael Van Veen'
}

Solves the following problems:

• People have different ideas about what metadata fields are
• PKG-INFO fields have their own logic separate from metadata global usage, further complicating the namespace.
• A good API (and in turn, a good UI) should respond to all these different types of metadata clearly.

Idea: make response special metadata type (attrdict)