Adding a DatasetFolder class.

[freud14]

Hi,

I split up the ImageFolder class to create a DatasetFolder class so that any dataset split in folders can be loaded. I also added unit tests for ImageFolder.

Thank you.

[soumith]

lgtm, thanks @freud14 !

[vfdev-5]

@freud14 I wonder whether it is necessary to upload images a*.png and b*.png. Correct me if wrong, but it seems to me that the content of images is not compared in the tests. If it is the case, the test script can simply create files in a temp folder, run tests and clean up.

[fmassa]

@vfdev-5 I don't think it's necessary to have several similar images for testing, only one or two would suffice I think. But there is no harm in keeping them now