FIX #143 - issues with labelme2coco Dataset in MaskRCNN custom train

[mabu-dev]

This fixes #143

Fixes include:

1. Make load_dataset compatible with labelme2coco.convert()
2. Ensures that labelme2coco.convert() respects the 0 class ID reservation and avoid throwing:

if class_id < 1:
print(
'Error: Class id for "{}" cannot be less than one. (0 is reserved for the background)'.format(class_name)
)

3. Updates docs for custom train example