Two select_triplets functions with performance improvement

[scotthong]

Hi David,

I added two select_triplets functions and here is a quick summary of the changes.

1. Renamed the origin select_triplets to select_triplets_orig
2. The version 1 of the implementation using a pre-calculated distance matrix (select_triplets_v1).
3. Further improvement over Add data augumentation #2 with additional 15% of performance improvement. This is the default one using select_triplets function name.

The latest implementation improves another 15% over the first one with only distance matrix.

Please review the the changes, especially #3 that might not have the same rules as the original one.
These changes do improve the performance by almost 3x compared to the original one when running on my computer.

Thanks,

Scott Hong

[scotthong]

I've validated the two functions added to facenet.py. The second version doesn't work as well as the original implementation. The select_triplets_v1 is the one that has the same logic used in the original implementation. This version can achieve the same LFW training accuracy as before (~0.92). I've cleaned up the three into one in my latest forked master branch.

[davidsandberg]

Looks good! Thanks alot!!