make some spacing changes to the main page

there are other things that i want to do, but not being a designer/in charge of this project, i'll list some of my ideas here:

- there's too much info (IMO) on the main page. i think it should be a pithier elevator pitch. if it could be boiled down to "data model", "operation", and "visualization" i think that would go further in making the initial "sale" ("what is it? how does it work? how can I use the data?")
- needs color. i'm terrible at this. but it's a lot of black.
- a lower section with some more detail, about the main 3 points. couple pictures per section adding visual info, e.g. picture of an example data set to show the dimensionality, a gif of a terminal running a compile and scp'ing the final single binary, and a graph showing what visualization can look like
i'm partial to the smaller text. i think both fonts (open sans and avenir) are easy to read at the smaller size, and at the larger size they start to compete with the "prometheus" main title off to the left. i do agree that in 'open sans' the letters are thinner, so it does start to get small. i think the color contrast mitigates this, and the menu is in a standard position, so i'm not worried about people missing it. if you feel strongly about bumping up the size i say we ditch avenir in the title, because i think it gets too big at the original font size. your call!

open sans:
![image](https://cloud.githubusercontent.com/assets/1398104/5526254/c7df8e04-89f1-11e4-9cf5-dff500436753.png)

avenir:
![image](https://cloud.githubusercontent.com/assets/1398104/5526259/d6e24248-89f1-11e4-8202-3f2253c61798.png)