Minor tweaks to VocabTree2

Dear Noah,

I've been playing a bit more with VocabTree2, and I've made a few small changes to my branch. Perhaps they are useful for you as well. Here's what I changed:

- VocabTree.h: Ensure initialization	d1c7f84
  
Here, I noticed that when I initialize a VocabTree object, the number of nodes was undefined, rather than set to 0 as expected. I initialize other members as well along the way.

 - VocabTreeIO.cpp: Hide debug statement	7d720e3

Here, a statement was being printed each time the matching functionality was invoked, and since that potentially can happen many times, probably such a statement should be hidden.

 - Make some vars more consistent	 87209ba, and one more tweak to usage a8809cd

Here, I noticed that the code has slight inconsistencies. Following the name and intention of the VocabBuildDB executable, I think it may be helpful to distinguish between a tree and the database created from the tree.