New Rule: no-lone-blocks (fixes #512)

Removed extra down-traversal in favor of looking up one level at
ancestors and changed config from warning to error to incorporate
PR feedback.
Merge pull request #572 from btmills/no-lone-blocks

New Rule: no-lone-blocks (fixes #512)
New Rule: no-labels (fixes #550)
Merge pull request #577 from eslint/no-labels

New Rule: no-labels (fixes #550)