Removed ambiguity in comment.

Thanks to Sebastion Vigna for pointing this out.
Extracted top_k_greedy from wt_int and added

navigation functionality to wt_int. A node_type
was added to wt_int and methods
 - root()
 - is_leaf(v)
 - expand(v)
 - exapnd(v, r)
 - expand(v, vector<r>)
 - empty(r)
where v is a node and r is a range inside a node.
push -> emplace
Moved intersect to new header wt_algorithm.hpp
Moved range_type and range_vec_type.
Fixed range_type.
Merge pull request #141 from simongog/wt_housekeeping

Wt housekeeping