First quick implementation of BRIEF Feature descriptor

After reading the BRIEF paper (http://cvlabwww.epfl.ch/~lepetit/papers/calonder_eccv10.pdf) for about 3 times, I found the BRIEF feature descriptor surprisingly simple.

Currently, this code includes many `for`s and `if`s, so please suggest if some numpy optimization can be applied anywhere.

TODO :
- [ ] Add implementation for the `mode='normal'`
- [ ] Add tests, doc, example