Move to Virtuals

[jcspencer]

In reference to #10, one of the aims for v1.0.0 is to move from patching MongooseArray and Schema and to instead use Mongoose's Virtuals. This would allow mongo-relation to be used as Mongoose Plugin, instead of monkey-patching via a require call.

This would involve moving the following methods into Virtuals on Schemas:

• belongsTo
• hasManyBelongsToMany
• hasMany
• hasOne

As well as adding .create() methods to each relationship field, wether it be an array or single document.