Switched broken pypip.in badges to shields.io

Adds support for 'response-language' in request

Implementation of https://github.com/ProjetPP/Documentation/commit/81848b3f3f3c853aaa4751a7f272d00552429e97

In communication objects, raise appropriate type of exception if attributes do not validate.

ie. TypeError if they are not the right type (consistent with nodes),
and AttributeNotProvided if an attribute is missing.

Add optional TraceItem.times attribute.

Add equality between a singleton list and its element. Closes GH-45.

Return NotImplemented when testing equality between TypedAttributeHolder
instances with different types.

Factorize serialization-related code.

Introduce Nth node.

Introduced in https://github.com/ProjetPP/Documentation/pull/59

Add Python2.7 support.

Split AbstractNode into four classes.

AttributesHolder -> TypedAttributesHolder -> SerializableAttributesHolder -> AbstractNode

AttributesHolder and TypedAttributesHolder: theoretical part of the datamodel specification
SerializableAttributesHolder: serialization part of the spec
AbstractNode: utilities for making operations on trees.

Implement Triple.inverse.