Git resource

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, :root_uri_path => '')
```

It returns HTTP 403 Forbidden on write methods (eg. put, delete, mkcol)