Add exit code to return value

If `ec` is not zero, exit with `ec` value
Add optional flags to make and (N)ant
Add multiple target execution (hack)
Update documentation for new target passing
Merge branch 'add_return_code_exit'

Conflicts:
	xnt/xenant.py
Merge branch 'add_optional_flags_to_build'
Merge branch 'add_multi_target_support'

Conflicts:
	xnt/xenant.py
Increment Version -- Add release notes