More models and some bug fixes

• Ability to switch image backends between PIL and accimage
• Added more tests
• Various bug fixes and doc improvements

Models

• Fix for inception v3 input transform bug #144
• Added pretrained VGG models with batch norm

Datasets

• Fix indexing bug in LSUN dataset (#177)
• enable ~ to be used in dataset paths
• ImageFolder now returns the same (sorted) file order on different machines (#193)

Transforms

• transforms.Scale now accepts a tuple as new size or single integer

Utils

• can now pass a pad value to make_grid and save_image