fix issue #27

[Ze-Yang]

1. record the exact instances when constructing meta-data set, which will be stored in instances_shots2014.json.
2. get rid of 3x shots for base classes

fix #27

[YoungXIAO13]

The original sampling code is inspired from Meta R-CNN, which could indeed cause a problem as you mentioned.
That's why I've done a double-check using the json files provided by TFA.

The modification is indeed very clear and useful, I've merged it.
Thanks!