Multiple config sources

@kmcphillips @christopherlobay 

ShopifyApp::Configuration works the same way it did before except for a few differences:

* Multiple configuration sources
* New order of precedence:
    1. ENV variable
    1. Configuration set in a Rails `<environment>.rb` config file
    1. Configuration loaded from `<environment>` key in `shopify_app.yml`
    1. Configuration loaded from `common` key in `shopify_app.yml`
    1. Defaults to empty string
* Future proof: just add new configuration options to the `VALID_KEYS` array

Seemed simpler to do it this way instead of involving another gem. There's some magic but it's literally all there instead of in some gem.

/cc @jeromecornet