Moved UUID functions to BakerAPI
Encapsulate all API calls into BakerAPI
Better BakerAPI error management for status != 200
Added support for placeholders in API URLs

This will make it possible to define something like
#define NEWSSTAND_MANIFEST_URL @"http://example.com/:app_id/issues"

The :app_id placeholder will be automatically replaced with the app_id.
Same goes for :user_id.
DRYed up GET/POST helpers in BakerAPI
Refactored error management in BakerAPI