Airline: Disabling Powerline Fonts and Updating README

Makes the desired changes mentioned in comments of Issue #413.

Before merging there are a couple of items to address:

1. I added the Airline section at the end of the list of plugins. Perhaps it should be sooner so people are more likely to see it? I feel like the config loses some of its pizazz without the powerline symbols enabled.
2. I am hosting an example image on my own imgur account but it feels like it should be on the same account as the other screenshots. If you upload that image and give me the new link I will gladly change the README.