prettified the disconnectivity graph

Thanks for the great library, guys. I've made a few improvements (I hope!) to the disconnectivity_graph module. Here they are:

- An artifact could arise when a minimum's energy was above the energy of the bin level minus the energy offset, causing the vertical portion of the line to go up instead of down. I fixed this by forcing a single line to be drawn directly to the minimum's energy, instead of to the offset and then the minimum's energy. This appears to be how this issue is solved in Wales's Energy Landscapes. It's very clear in the middle of the d. graph on page 265 of that book.
- I slimmed down the lines, added a y-axis spine, shifted the plot to the right slightly, put the ticks on the outside, changed the aspect ratio, and set the background color to white in order to closely match the style of Wales's book. 

Here's an example result:

![tree](https://f.cloud.github.com/assets/929443/54409/8033c0f2-5a91-11e2-9f3d-4279a8b2e14d.png)

Thanks again for the library, you guys saved me a lot of trouble making these figures. :)