If an empty directory is overwritten we will always get
ENOTEMPTY as the "empty" directory will still contain gocryptfs.diriv.
Handle that case by removing the target directory and trying again.
Fixes issue #10
Move all the intelligence into the new file address_translation.go.
That the calculations were spread out too much became apparent when adding
the file header. This should make the code much easier to modify in the
future.
