ENH: refactor reset() to set_axlim() [also a simple box-zoom plugin]

This refactors ``reset`` to a function ``set_axlim``, as discussed in #90. In doing this, this PR simplifies some of the code by factoring-out a utility routine for interpolating dates.  I've also done a small [PR to d3 itself](https://github.com/mbostock/d3/pull/1779) to add this capability.

Edit: to show how ``set_axlim()`` might be used in practice, I added a small plugin example showing a box zoom tool.

There are still some issues with propagation of axlim settings, so I have to figure out what's going on there...