Use composer for autoloading, fixes T197

Summary:
- remove Pimple from `src/`, install with composer
- remove custom autoloader, register with composer

Test Plan:
- run `composer install`
- try app... if it works, nothing broke

Reviewers: rjmackay

Reviewed By: rjmackay

Subscribers: dkobia

Maniphest Tasks: T197

Differential Revision: https://phabricator.ushahidi.com/D47