checkpoint proper var expansion
fix and test var expansion in a single file
fix include handling
implement scanning for roots if no default given
lint
add pool.py to build.ninja
Merge branch 'master' of https://github.com/dpranke/pyn

Conflicts:
	main_test.py
linting
fix bugs in host_fake and switch main_test.TestArgs to use a fake host
lint
clean up TestArgs to reuse CheckMixin
clean up integration_test.IntegrationTestArgs, lint