`LabeledDataset` merge directory structure

[mattphotonman]

The directory structure of a merged LabeledDataset gets messed up when using relative paths.

from eta.core.datasets import LabeledImageDataset

small_train = LabeledImageDataset("relative/path/to/manifest_small_sample.json")
small_val = LabeledImageDataset("relative/path/to/manifest_small_val_sample.json")
small_train.merge(small_val, "small_catdog_dataset/manifest.json")

$ ls small_catdog_dataset/
data  labels  manifest.json  small_catdog_dataset

$ ls small_catdog_dataset/small_catdog_dataset/
manifest.json

[mattphotonman]

Closed by #295