travis: run CodeSniffer to detect undefined variable usage
add <properties> tag to ruleset
restrict codesniffer warnings to undefined variables
Merge branch 'master' into codesniffer
ignore codesniffer warnings in core.php

some are caused by using variables defined in wp-includes/version.php

and some are caused by using extract(), which isn't that evil when used
with EXTR_SKIP
make codesniffer ignore Spyc library
ignore undefined variable warnings in post.php
fix undefined variable warnings in wp-settings-cli.php
ignore undefined variable warnings in Runner.php
extract validate_role() helper method
fix undefined variable warnings in user.php
move codesniffer parameters to ruleset.xml
enable DisallowSpaceIndent sniff
enable EndFileNewline sniff
Merge pull request #818 from wp-cli/codesniffer

Run CodeSniffer in WP-CLI Travis builds