VocabLib/include/DirectIndex.hpp
* Added method for looking up an image on the index.
* Changed save/load methods signature from receiving a filestorage to a
filename.

VocabLib/include/VocabTree.h
* Enabled on the vtree base class the direct index save/load methods.
* Fixed methods documentation.
* Moved direct index filestorage handling from VocabTree to DirectIndex.

VocabLib/src/DirectIndex.cpp
* Moved direct index filestorage handling from VocabTree to DirectIndex.
* Changed structure of the text file format to a numeric array.

VocabLib/tests/.gitignore
* Ignored test executables.

VocabLib/tests/DirectIndex_test.cpp
* Implemented tests for the DirectIndex class.