-merged with chad's 2.0 branch, all unit tests passing now
-rez-exec renamed to rez-interpret
-added new unit test, 'context'
-made shell detection more robust, found a breaking test case
-misc bugfixes
moved contrib/ to vendor/
-added vendor/argparse.py, removed argparse as external requirement
-added ResolvedContext.which(), this is used in build sys plugins
-updated cmake/bez buildsys plugins so that they search for the builder
binary in the build env first, then fall back to calling env. This maximises
chances of finding cmake (for eg) without necessarily needing to create a
cmake rez package
-added timestamping back again. Compatible with rez-1 and rez-2 (which stores timestamp in the new release.yaml file)
Merge branch 'nerdvegas/2.0'

Conflicts:
	setup.py
	src/rez/packages.py
	src/rez/resources.py