Adds queue data structure and tests.

Queues exhibit FIFO or first in first out behavior.

With dequeuing if there is a single node in the list
we ensure the last instance variable is updated
to nil.
Refactors nil tests in methods to be more
idiomatic.

Adds #peek method to view first node data
in queue without dequeuing
Merge pull request #6 from hwhelchel/queue

Adds queue data structure and tests.