Add IsEncrypted() method to File struct

[belyalov]

It is an easy way to determine if file is encrypted - useful when you need to e.g. find and do something with password encrypted archives.

I tried to keep it simple and move this code out of data path - to make it seamless for everybody who does not care about this functionality.

[LinkLeong]

This function is very useful, I hope to deal with it as soon as possible