Viewport resizing

[corranwebster]

This adds logic to the Viewport component (and some auxiliary changes for the Scrolled component) that allows move familiar scrolling conventions.

Viewports can now specify horizontal and vertical anchors for the view which indicate where the initial position of the viewport should be relative to the component, and the fixed point when the bounds of the view change.

In addition the unimplemented stay_inside option is now implemented: when changing the bounds of the viewport the view will try to stay within the bounds of the viewed component, if possible.

This PR is built on #205, since it makes the interactive testing easier, but is technically orthogonal to it.

Tests included.

[corranwebster]

Hmmm... we seem to have broken 2.6 builds with new traitsui version.

Do we still care about Python 2.6.

[mdickinson]

Do we still care about Python 2.6.

I would vote yes: recent versions of RedHat still seem to be using Python 2.6 as their system Python, and I'm fairly sure we still have some customers who are Python 2.6. How hard are the failures to fix on 2.6?

[corranwebster]

Not sure - we are no longer testing on TrauitsUI on 2.6 with the move to support Python 3. I'll look into it. If we support 2.6 we should be testing everything on it - breakage is too easy otherwise.

I think that there are significant wins for supporting 2.7 and 3.3+ only.

FWIW, Redhat 7 ships with Python 2.7 and has been out for over a year at this point.

[corranwebster]

With #205 ready for merge, I think that this PR is as well.

[corranwebster]

Looks like the problem was something transient with Travis trying to get traitsui. Re-running the tests, everything is passing.