EXR: Added sxr and mxr as possible extensions
fixed compiler warnings
string_ref helper class encapsulates a char* and length without ownership.

This is really helpful for passing string arguments to functions that
need to examine the contents of a string but do not themselves need to
own it.  You can uniformly pass any of char* or std::string or a string
array.  it will do so without allocation or copying.

This is very similar to the string_ref that has been proposed by the
C++0y standard (but for which we can't count on uniformly available
compiler support for many years).
use string_ref where helpful in Strutil and ustring.
string_ref amendments to ustring construction
use string_ref for filesystem
Add missing include to fix compilation error.
Fix misleading copyright noticed (must have been cut and pasted wrong)
Excise the remnants of TBB
Add compression level and compression strategy to pngoutput
Get rid of deprecated cruft. Nobody cares about Gelato compatibility.
Alternate webp reference output for newer version of webp library.
OpenEXR: for older OpenEXR versions, avoid the headers and types that don't exist
including StringVectorAttribute, etc.
Make IB iterators return valid black pixel data for missing tiles.
It's a lot better than crashing when you dereference a NULL pointer!
Bug fix oiiotool --origin could crash with ImageCache-backed images.

In oiiotool's set_origin action, the naive resetting of the spec's x and
y values is not safe if the IB is backed by ImageCache -- the mismatch
of the image origin (which determines the tile boundaries) between what
the IC thinks and what the IB thinks leads to grief.  The fix is that
prior to resetting the image origin, cached images need to have a read
forced to bring them into memory.  Images fully in memory already are
fine to just diddle the origin value.  This does not necessarily make
--origin more expensive, since oiiotool alread does the force-read on
reasonable-sized images (for speed) and only caches really big images.
SPI-specific internal build instruction changes from Sosh
CHANGES
SPI customization: Allow specification of PYVER from the command line
SPI customizations: fix incorrect linkage of python libs to lib/libOpenImageIO.so
We only want to link against python for the python module python/OpenImageIO.so
Clean up some other Python handling in SPI's custom build setup, including being
able ot specify a different python verion with the PYVER environment variable.
Bump dev version