Resurrected bendavis78's modified_time support to aid with collectstatic

I need to use collectstatic instead of syncstatic because I'm using custom post-processing only supported by collectstatic. Implementing the modified_time method on the storage greatly helps with the performance of that because it'll only upload files that are newer than the ones on the server.

@bendavis78 implemented this a year ago, I just pulled it into the current codebase. It works nicely, please merge!