For #9.

See bazil/fuse#89 for more details.

Caused by jacobsa/bazilfuse@47700022b94da7c946b1f5f8af8621b2a32f4762.

For #9.

Now that I have some references to cite.

For GoogleCloudPlatform/gcsfuse#60.

Added tests that confirm behavior of "real" file systems, then made memfs make
those tests pass, too. Doing this by returning EEXIST appears to work on both
OS X and Linux, but I can't find documentation anywhere that says this is what
is expected.

More:
    https://github.com/osxfuse/osxfuse/issues/209
    http://stackoverflow.com/q/30364838/1505451
    http://sourceforge.net/p/fuse/mailman/message/34130996/