Annotate models + Order scope bug

[sohaibbhatti]

This Pull Request serves two purposes

Adding Annotate Models

In the code base, all models and specs have their attributes and corresponding datatypes in the form of comments. This makes development for developers easier by serving as a visual cue.

Fixed a Bug in the order completed scope

Prior to this, the Order.completed scope was returning the following SQL.

   SELECT "orders".* FROM "orders" WHERE (token != '' OR token != NULL)

The SQL is incorrect because logical operations on NULL will always result in a NULL. i.e. (NULL = NULL would yield a NULL). It has now been fixed to

   SELECT "orders".* FROM "orders" WHERE (token != '' OR token IS NOT NULL)