Remove before adding to mix map.

Because it would be *crazy* to want to overwrite an existing map entry, and it's always better to do moar map lookups.
Separate Unity 4 and 3.5 examples.
Fixes needed for Unity 3.5.

Unity 3.5 is pretty terrible. :(
Example project instructions.
More text.
Meta files for Unity 3.5.
Merge branch 'master' into monogame