NEXUS-6537: Do a move when possible

Perform real move when possible. Same limitation stands on Java7 Files class as before (move on non-empty directory is possible only if on same partition, subtree does not have to be moved), but this time there are means to detect does this stand or not.

Issue
https://issues.sonatype.org/browse/NEXUS-6537

CI
http://bamboo.s/browse/NX-OSSF78
By experimenting (look here https://github.com/cstamas/nio2) on OSX I get mixed results, and I'd just leave out atomic move for now. Later we can experiment more, as I expect that test output vary from OS to OS.