Pic2vec

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* Limited Keras version to pre-2.3.0 to fix issues from Keras' breaking changes

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* Updated version of Trafaret to a non-beta version
* Updated keras to 2.2.3 or greater
* This library upgrade changes prediction consistency across past versions of pic2vec.
ResNet50 is the model that has changed the most, due to changes in implementation.
Other models have small floating point changes, but still pass np.testing.assert_allclose tests.

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* Updated version of Pillow to 5.4.1, in order to support Python 3.7
* Updated the README

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* Added test coverage and increased error checking
* Changed default csv name
* Changed `image_column_headers` to `image_columns` everywhere
* Updated examples
* Updated version of scipy to 1.1 and numpy to 1.15

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* Updated the notebook example
* Some code cleanup

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* Lots of code cleanup
* Changed new_csv_name argument to new_csv_path everywhere for consistency
* Removed '_full' from the saved csv_name for the full dataframe. Features-only csv still has
'_features_only' in csv name.
* Added '_featurized_' to saved csv names
* Removed new_csv_path as argument to functions that do not actually require it