Add Sniff class to check for variable occurances

This handles:
- Plain variables, ex: $foo
- Object vars, ex: $foo->bar
- Array members, ex: $foo['bar']
And it supports searching for all of the previous within double quoted strings as well.
Flag use of wp_users/usermeta #78
Merge branch 'develop' into variable-restrictions-sniff
Merge branch 'develop' into variable-restrictions-sniff
Make usermeta update test more sensical; add positive test
Merge pull request #108 from WordPress-Coding-Standards/variable-restrictions-sniff

Add variable restriction sniffs