ERB --> Haml templates & lots URI/HTML escaping

I took the opportunity to learn some more about how Sinatra hangs togher
and bring some things in line with a more convential/sensible way of
doing things.

Tested-by: Mark Somerville <mark@crocodile.is> [Arch, MRI 2.1.1]
Signed-off-by: Mark Somerville <mark@crocodile.is>
README: minor update - Ruby >= 2.1.0 is required

Signed-off-by: Mark Somerville <mark@crocodile.is>