Git resource

A simple resource for a git repository. It allows reading of the HEAD tree using [grit](https://github.com/mojombo/grit).

Usage in config.ru for example:
```ruby
require 'dav4rack'
require 'dav4rack/resources/git_resource'

use Rack::CommonLogger
run DAV4Rack::Handler.new(git_dir: 'test/fixtures/smallgit.git', resource_class: ::DAV4Rack::GitResource)
```

It returns HTTP 403 Forbidden on write methods (eg. put, delete, mkcol)