Also cleaned up the general mkdir functionality.

Tested: on OS X.

Tested that a command line like: ./flatc -j -o foo/bar/baz tests/monster_test.fbs
generates files in foo/bar/baz/MyGame/Example/*.java which previously didn't exist.

Windows code was previously tested but has been moved, so needs to be tested again.

Change-Id: Iee943121b3be4f92961a3ec94c2cb578165f114a