Increased global font size, increased font weights and added favicon

@jinpa - is it ok if I assign this to you? It's a graphics thing... Also about the weight, we looked at it with David and that change (300 to 400) seemed to work for him...

Like the title says:

- I increased default font size to improve legibility and also impose a good baseline for font rendering.
- I changed the weight of the referenced font (Source Sans Pro) from 300 and 600 to 400 and 700
- I also added a favicon...