Review: OIIO namespace chanages

Some of you have seen on oiio-dev that Jeremy Selan has proposed a patch to fix up some longstanding OIIO namespace issues, namely to put some stray namespace-less helper classes (TypeDesc, ParamList, ustring, etc.) in the OpenImageIO namespace once and for all, as well as to clean up and centralize some of the versioning and custom namespace selection.

The attached patch alters OSL so that it builds properly with both the old OIIO as well as the proposed new one.  To make it work for both old and new, it's a little uglier than I'd like.  But fundamentally, it's just rearranging some #include file ordering, and conditionally adding the occasional 'using' and 'namespace=' directives that are needed once those helper classes (which we use extensively in OSL) get in their proper namespaces.

Some of the ugliness can be removed down the road, once we're in a state where all OSL users are are on OIIO 0.9 or above.