ofFbo working with ofTextures created externally.

the original PR #2596 had some merging issues so this new PR was created.

@arturoc, regarding your last point.
i think it should be safe to include this in 0.8.1
ofFbo functionality is still exactly the same and internally the ofFbo still does the same thing,
only thing that changes is that ofFbo now exposes another method which people can use to add ofTexture objects externally.

can't think of anything else that could cause issues... should be pretty safe.