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/

For GoogleCloudPlatform/gcsfuse#12.