Rule: one-var

The `one-var` rule disallows multiple variable declarations per function scope.

function foo() {
  var bar;
  var baz;
}

See: http://jslinterrors.com/combine-this-with-the-previous-var-statement/
Removed unnecessary rules.js test.

Two PRs were merged that both introduced test code that tested the same piece of functionality. I'm removing one.
Refactored `one-var` rule.

The `one-var` rule now pushes to a function stack and keeps track of variable declarations for each node on the stack.
Merge pull request #206 from iancmyers/rule-one-var

Rule: one-var