Tweaks a few style parameters

Based on @astrofrog's comments in #46.

This does __not__ do the following:

* Make more of the method names links to the API docs (@astrofrog)
* Use a nicer default set of matplotlib parameters (@keflavich)

Could you make those as new issues? I'm punting the matplotlib style stuff until matplotlib 1.4 comes out because the style stuff has now been merged into mpl (https://github.com/matplotlib/matplotlib/pull/2236).

This PR also adds some text about how to leave feedback about the tutorials, so it is related to #50 as well.