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
Fixed bug in IECoreRI::Renderer camera editing.

It was incorrectly storing m_lastCamera (the primary camera) even during edits, meaning that if cameras other than the primary camera were edited, it would lose track of the primary camera. For common sequences of edits (all cameras in order, primary last), this meant the main world camera would never be updated.
Merge pull request #351 from johnhaddon/multipleCameraEditsFix

Fixed bug in IECoreRI::Renderer camera editing.
Bumped alpha version and updated Changes.