Changes to data structures to fix bugs: issue #6

[tomcostello]

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I noticed some bugs in the implementations of the first few data structures in the algorithms course. In LinkedList.js the tail was not set if the element added was the last element. In Queue peek should look at the last element of the array, the next one to be popped, not the first element. In Queue, one of the tests was wrong, as the elements are stored in the reverse order in which they are added, as they are pushed on the front using unshift.

The one arguable change is in Stack.js where I think the return code should be null, when you pop or peek and empty stack, but I suppose some people could think the opposite.