Add Code Sniff to catch imports that are not used

[justinyost]

Display warnings on importing a class that isn't used in a class.

Possible lead on solutions: squizlabs/PHP_CodeSniffer#801

[beporter]

More specifically, possibly this: http://cgit.drupalcode.org/coder/tree/coder_sniffer/Drupal/Sniffs/Classes/UnusedUseStatementSniff.php?id=06ea52ef66833590df592e9fdb23393352caf5d1