MD5sum deep compare

[esper80]

I suggest to add a function for deep compare.
Not only detect files changed by date comparision, but also with md5sum comparision.
It should be very slower, but it is a more accurate comparision that could be useful sometimes.

[RiseFly]

Sorry, we have not found a method that has high performance to compare the file contents. We think that comparing file time and size is a good enough method.