Add adhearsion-i18n as a dependency
Replace plugin configuration-supplied prompts with adhearsion-i18n YAML
Make all voicemail prompting use i18n
Remove MailboxPlayMessageIntroController, it's not used
Make IntroMessageCreator into a module
More adhearsion-i18n conversion of CallControllers
Fix specs
Load voicemail after adhearsion-i18n
Add the template to the I18n load path

This will load templates/en.yml, but at lower precedence than anything loaded by adhearsion-i18n. I18n loads the translations from the load_path in sequential order, so the closer to the end each file is, the smaller the chance it will be overridden.
Address Hound's comments
Merge pull request #38 from adhearsion/feature/adhearsion-i18n

Convert prompting to adhearsion-i18n