Extract Conflation Class around YAML Handling

Per @anfleene's suggestion, this moves logic of reading YAML files and applying configs into a stand-alone class. This makes things much cleaner, and makes it easier to begin reading from more than just YAML files if we want to.


QA Plan
-----------
Same as before.

1. Launch a Rails console for any app.
2. `$LOAD_PATH.unshift File.expand_path "~/workspace/conflate/lib/"`
3. `require "conflate/conflator"`
3. `Conflate::Conflator.new(Rails.root.join("config"), Rails.application.config).perform`.
4. Verify that Rails.application.config got some more keys for each YAML file you have in config (e.g., assuming you have a database.yml, Rails.application.config.database should contain the hash of contents from database.yml).