Cleaned up history by removing entries for non-existent images and im…

[AEtheve]

…plemented disk cleanup when deleting from history.

[divamgupta]

Thanks for the PR.
I see one case on which it might fail.
Say you have in image X, then you use X for img2img, then you delete X from history. In that case it should not be deleted from the disk as it still needs to be there in history.

[divamgupta]

Maybe a cleaner solution is followed:
On startup, do ls on the image folders, and delete the images which are not present in the history object.

[Leland]

Resolves #121