Add the new class Attributes holding multiple Attribute(s)

Add the new class Attributes holding multiple Attribute(s)

Attributes represents a semantic collector of Attribute(s) and replaces
the anonymous Map<String,Attribute>. This class will be returned by
TreeWalk.getAttributes(). It offers convenient access to the attributes
wrapped in the Attributes object. Adds preparations for a future
Attribute Macro Expansion

Change-Id: I8348c8c457a2a7f1f0c48050e10399b0fa1cdbe1
Signed-off-by: Ivan Motsch <ivan.motsch@bsiag.com>