FIX: Uncaught TypeError: Object #<Object> has no method 'toTree'

Use the correct 'this' value when attempting to pull hrs out of
lists.
Minimal test to expose scoping bug.

This test fails in older version in which the call to horizRule
was incorrectly scoped.
Merge pull request #192 from josephturnerjr/horizrule

FIX: Uncaught TypeError: Object #<Object> has no method 'toTree'