Integrate other feature descriptors into loop closure framework

[stephanietsuei]

The current bag-of-words loop closure framework only supports BRIEF descriptors. It should support all the feature descriptors supported in the Tracker, including those implemented as part of issue #39. For each descriptor, this requires

1. Creating a vocabulary for DBoW2 for each new descriptor, like cfg/ukbench10K_FASTBRIEF32.yml.gz
2. Implementing an interface to DBoW2, as is done in src/fastbrief.h and src/fastbrief.cpp

The interface for binary descriptors should use a form of int * as the type for TDescriptor for speed. (Do not use std::bitvec, as is done in the DBoW2 demo for BRIEF descriptors.)