Update colors (yellow --> blue)

Here we change the color scheme from yellow to blue. All colors are
based on the blue used in the new H2GIS logo: #0092ff.

                               Old      New
Navigation                     #fc0     #4db3ff
Navigation-Hover / Text-Hover  #f90     #0092ff
Text                           #fc0     #87ceeb
Quickstart background          #3f1f1f  #00223b

See http://www.colorhexa.com/0092ff

Note also that _includes/css/style.css was replaced by a symbolic link
to css/style.css to avoid having to maintain two separate css files.
Change separator color (red --> pale yellow)

I'm not sure this is the color I want.
Add H2GIS logo (this isn't the right one)

Thanks @gpetit for the logo.
Fix H2GIS logo

The problem was that I didn't have the right font on my system. This svg
uses a vectorized version of the font. If you want to recover the
original font, look in 396949b.
Update favicon
Change quickstart background color

The dark blue looked bad.
Change download button colors
Add download button hover effect
Add hover transition effect to dl button