Implement thin layer for datastore abstraction

 This abstracted datastore class should help with implementing multiple
 datastore backends for boomerang-express and allow support for more
 environments.
Add unified error codes to simplify error handling
Add mongodb datastore backend to reach compatibillity with previous system
Factor out middleware configuration and instantiation to own middleware
migrate app.js to new datastore and middleware
add NeDB datastore backend
Migrate routing to new datastore abstraction
update configuration to reflect new datastore backend
add nedb to dependencies
make an example config
make Backend more robust to configuration or system failuree
make backends more stable and failure-resilient
add unit tests for backends
add developer documentation on backend storage
ignore local test-config
remove uncessesairy whitespace
fail start on errors
fail when no backend is defined
add new devDependencies for tests
Merge branch 'backends-nedb' of github.com:andreas-marschke/boomerang-express

Merged .gitignore and dependencies in package.json from backends-nedb and master