do not merge: pasted from keras pull request

[ahundt]

Can't do this right now, but there may be some changes worth retaining here from this pull request based on this repo: https://github.com/fchollet/keras/pull/4718

I just pasted the changes over here so the information isn't lost without evaluation or merging, hence the "do not merge" in the title. It looks like some of the code changes and comments will be useful, but others like combining the files is less so.