json parser for annotation parsing

[chadrick-kwag]

• added annotation json parser.
• added --annotationformat option in flow command. If this option is not specified, then it will by default use the PASCAL-VOC-XML parser which is the legacy method.
• added /docs directory in root and added doc on json format for annotation

for the json format, check out the docs.

[chadrick-kwag]

hmm.. the ci failure is an assertion error while loading the weights...

self = <darkflow.utils.loader.weights_walker object at 0x7fce80cb4dd8>
size = 16
    def walk(self, size):
        if self.eof: return None
        end_point = self.offset + 4 * size
        assert end_point <= self.size, \
>       'Over-read {}'.format(self.path)
E       AssertionError: Over-read /home/travis/build/thtrieu/darkflow/bin/tiny-yolo-voc.weights
darkflow/utils/loader.py:127: AssertionError

and other checks has passed

============== 1 failed, 5 passed, 1 warnings in 78.80 seconds ================

My PR has nothing to do with loading the weights so, can we just ignore this failure?

[abagshaw]

@chadrick-kwag CI failure does seem to be unrelated - probably due to some updated weights or something. I'll check into that when I have a chance.

[monoloxo]

is this PR helpful for training on coco dataset?
why can't i find the code about this in thtrieu/darkflow?
is there some way can training on coco dataset?cause voc is too small sometimes