Added dice_loss in keras_contrib/losses with docstring and pytest functionality

[soymintc]

- What I did
Added Dice coefficient loss function with documentation in keras_contrib/losses, and modified keras_contrib/losses/init.py accordingly
Added pytest functionality

- How I did it
Added dice_loss function in keras_contrib/losses/dice.py, and verified it using pytest.
Testing script is added in tests/keras_contrib/losses/dice_test.py

- How you can verify it

1. Ran pytest dice_test.py inside tests/keras_contrib/losses, which all passed
2. Uninstalled the package and re-installed keras_contrib, and re-did step 1, which had no problem.
3. Ran pytest in the base directory, which didn't result in errors for tests/keras_contrib/losses/dice_test.py

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This pull request fixes #484