Move Stripe API library to `libraries`.
Main index file: Add _LIBPATH constant.
Stripe: Update require path to use _LIBPATH.
Libraries: Add Mailchimp.
Mailchimp: Initial module commit.
Core: Language handler: Before parsing language file make sure it's valid.
Core: Language handler: Don't log invalid language files.
Libraries: Add Mandrill API wrapper.
Language menu: Selector: Complete rework.

Not having backend language in main language file caused problems
with selection. Problem was fixed by defaulting to first language in
cases where backend language is not available for selection.
Most of the selector code was rewritten.
Install script: Add tooltips.