Implement handling for gandi's multiple output formats

Gandi will return completely differently formatted output (depending on
the domain you request, as far as I can tell). This commit adds handling
for this situation.

- Added proxy template type (instantiates a template within itself, then
passes through all requests to that template instead);
- Added Yaml template type (adds dependency on Symphony/Yaml)
Merge remote-tracking branch 'origin/new_templates'