Bumped version to 9.0.0-a7 and updated Changes file
Merged: Fixed precision issue with computing the projection matrix for dsm depth remapping, which was causing deep holdout problems.

Conflicts:
	src/IECoreRI/SHWDeepImageReader.cpp
Merged: Applied same fix to shw writer as reader.
Hacked up constants in test case to pass again.

Conflicts:
	src/IECoreRI/SHWDeepImageWriter.cpp
Merge pull request #347 from danieldresser/shwPerpFix9

Applying shw conversion precision fixes to master
stopped IECoreMaya::LiveScene::readObject() returning null pointers
Merge pull request #349 from davidsminor/mayaSceneNullObj

stopped IECoreMaya::LiveScene::readObject() returning null pointers
stopped IECoreMaya::LiveScene::readAttribute() returning null pointers
Merge pull request #350 from davidsminor/mayaSceneNullAttrs

stopped IECoreMaya::LiveScene::readAttribute() returning null pointers
Cleanup display driver.
Merge remote-tracking branch 'upstream/master' into IECoreAppleseed
Updates and cleanup