[Feature Request]Use magic word to allow element calculation

[LaoshuBaby]

For example, define a __add__() function to allow user merge 2 node/way/relation

And have such logic:

if A.type == B.type:
    if A.type == "Node" and B.type == "Node":
        C = Node()
        if A.pos != B.pos:
            C.pos=avg_pos(A.pos, B.pos) #Need geography lib support
        for ki,vi in A,tags:
            for kj,vj in B.tags:
                 if don't conflict:
                     C.tags.append(ki,vi)
     if type == "Way":
        pass
     if type == "Relation":
        pass
else:
    throw exception("Type for A with B don't match")

This also should offer a method called add(A:Node, B:Node), like a=Way\a=a.reverse() mentioned in #5