Support environmental variables in config.
Test env variables in config
Unset env variable after test
Merge branch 'versions/0.7' into dev

Conflicts:
	CHANGES.txt
	CMakeLists.txt
	cmd/hekad/main.go
	docs/source/conf.py
	pipeline/config.go
Merge branch 'expand_envs_config' of github.com:ecnahc515/heka into ecnahc515-expand_envs_config

Conflicts:
	CHANGES.txt
Protect globals.Stopping with RWMutex.

Make globals.Stopping unexported and wrap writes/reads with methods that lock
on a RWMutex.
Adjust variable/method names