Zip entry actions

[johkelly]

A more general implementation of #246 , motivated by the same desire to have shadow jars that play nice with rsync. This implementation pushes a list of Action<ZipEntry> into the ZipOutputStream code so that all processed ZipEntrys can be run against them.

Arbitrary actions can be written in the build.gradle:

shadowJar{
    modifyZipEntries { ze ->
        println ze.hashCode()
        ze.setName("mutatedName")
        // ...
    }
}

And the motivating action has a sugared form:

shadowJar{
    zeroZipEntryTimestamps()
}

I'm a coworker of the requester for #246 , and lifted some of his comments and test code.

[johnrengelman]

We're gonna need to make our own 1st class citizen for ZipEntry. We can't expose the apache classes as part of the public API for shadow because those classes are relocated during packaging.

But overall this PR looks good. I'll likely merge it this way and then figure out the stuff above.

[johnrengelman]

This work will go into a 2.1.0 build since it's adding new public API.