Add lib/test and guard
count, push, pop done
Fix guardfile; rename/delete files to get guard working
Add delete (works for list with one node)
Does not delete when data doesn't match
Delete middle node; delete head with a list of more than one node

The trick was to not set values straight to nil, but to attempt to set
them to the node after the node being deleted.  #next_node will return
nil if none exists, taking care of the previous behavior.
Add to_a; add last_node
Swap out infinite loops for while