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
Removed unused variable.
Merge pull request #142 from xosh/benchmark_wt

Benchmark wt
Fixed bug in range_search_2d
Fixed number of results in range_search_2d.
Added test for range_search_2d.
Merge pull request #144 from simongog/fix_range2d

Addressing issue #143
Moved quantile_freq to wt_algorithm

- moved empty(range_type) to wt_helper
- added size(range_type) operation
- moved topk_qprobing to doc_list_index_qprobing
Moved quantile_freq to wt_algorithm
Added raw member to allow serialization of

int_vector without size header.
Fixed compiler warning.
Fixed qprobing code.
Added tests for algorithms in wt_algorithm.hpp

Refined intersect (results are ordered according to
value if underlying WT is lex_ordered)
Addressed unused parameter warning.