Filesystem abstraction for storage

Summary:
Use the Storage facace for fileystem abstraction.

Removed 'app/' from default configuration value.

file_exists → Storage::has
file_get_contents → Storage::get

Test Plan:

• php artisan phabricator:projectsmap
• Test a GitHub payload request
• Test a Phabricator payload request

Reviewers: dereckson

Differential Revision: http://devcentral.nasqueron.org/D157