quantile topk_greedy topk_qprobe intersect added

added an implementation of the three wavelet tree functions. all
versions require an symbol order preserving wavelet tree. currently the
functions only work on wt_int.
remove copy constructor
add copy assignment operator
removed unnecessary constructors
Merge pull request #42 from mpetri/topk-wt-methods

quantile topk_greedy topk_qprobe intersect added