NumberAxis.showEndTickLabel(orientation, show)

#566: We could turn off tick labels on both sides, but we couldn't choose to only turn off ticks on a single side. This branch adds a function to change whether or not the end ticks show up on the right or left, top or bottom.

For example, on an axis with horizontal orientation (either `top` or `bottom`), you can turn on ticks only on the right with
```
axis.showEndTickLabel("right", true);
```

Close #566.
Might want to test the error-out functionality. In particular, are we intending it to throw an error if we have
```
var axis = new Plottable.Axis.Numeric(scale, "bottom");
axis.showEndTickLabel("top");
```
The above code throws the Error "Attempt to show top tick labels on a bottom axis", which isn't very clear. Maybe we should return `undefined` in cases where the user tries to retrieve invalid info, and error out if they try to set it?

@crmorford, can we get some input from a user's perspective?