Config refactor

- New InatConfig uses config.yml.example as the default config values
- Remove APP_CONFIG (merged into config.yml)
- Replace INAT_CONFIG[...] with new config class
- Made the config keys more consistent

Notes: I've removed the 'general' bit from config.yml as per [Ken-ichi's comment](https://github.com/inaturalist/inaturalist/issues/14#issuecomment-12682534).
I've also tidied up some of the config key names. For example:
- flickr FLICKR_API_KEY -> flickr key
- flickr FLICKR_SHARED_SECRET -> flickr shared_secret
- ubio UBIO_KEY -> ubio key

and renamed hoptoad to airbrake.

It's probably a good idea to have a look at [the changes to config.yml.example](https://github.com/inaturalist/inaturalist/pull/15/files#diff-47) to see what needs to be updated in your config.yml files.

I've left constants, such as SITE_NAME and OBSERVATIONS_TILE_SERVER, intact.

This fixes #14