Add validation for importer

closes #952
- moved api.js to api/index.js
- added api/db.js for import and export functions
- moved /ghost/debug/db/export to GET /api/v0.1/db
- moved /ghost/debug/db/import to POST /api/v0.1/db
- removed /ghost/debug/db/reset
- added validation for import
- added constraints object to migration