Overwrite flag does not work on OS X

[aelse]

In the rename function the force overwrite flag is tested after a call to os.Rename, which succeeds in overwriting a file on OS X but not linux. (incoming PR to fix)

[aelse]

Since opening this issue and PR #76 i see there's already the same fix in #64. It omits the os.Remove call) but this doesn't seem to be necessary (tested os.Create behaviour in OS X and linux and in both cases the file is overwritten). I'll close #76 and #64 looks fine to merge.