Add layout

To DRY up our templates, we have extracted the common bits into a
layout.haml file.

This required us to pull in sinatra-contrib, needed for `content_for`
and `yield_content`.

We also moved the CSS into a separate stylesheet. We'll likely use
Twitter Bootstrap to make things pretty soon.

Change-Id: Ib858e4430bb95834123aacc0a94d4bcfcfddd167
Reviewed-on: https://gerrit.causes.com/24890
Reviewed-by: Greg Hurrell <greg@causes.com>
Tested-by: Greg Hurrell <greg@causes.com>
Replace hand-coded <a> tag with Haml syntax

We'll eventually add a proper `#link_to` helper, but not yet.

Change-Id: I19899ef910ef0fcc2d4c318368a7ee4b5c36354c
Reviewed-on: https://gerrit.causes.com/24891
Reviewed-by: Greg Hurrell <greg@causes.com>
Tested-by: Greg Hurrell <greg@causes.com>
Rename twig to twigg

Alas, we've learnt that there is already a "twig" gem:

  http://rubygems.org/gems/twig

Funnily enough, it also deals with Git.

It is not widely used however, so we can just rename our soon-to-be-gem
to "Twigg", and it is unlikely to cause confusion.

Change-Id: I8f6b5965bc0ddb9efdf15eb9316b6941e991aa3d
Reviewed-on: https://gerrit.causes.com/24892
Reviewed-by: Greg Hurrell <greg@causes.com>
Tested-by: Greg Hurrell <greg@causes.com>