Implementing support for nested properties

[rafakato]

I've added support for declaring and attaching files in nested properties.

Defining nested fields:

schema.plugin(crate, {
    storage: storage,
    fields: {
       'nested.attachments': {}
    }
});

Attaching to nested field:

model.attach('nested.attachment', {path: '/path/to/file'})

I've added the dependency for lodash to manage get and set nested fields from string, I try to make it work with mpath that is the native from mongoose but the set method was not working as expected, it was not setting the nested property.

[coveralls]

Coverage decreased (-87.2%) to 12.821% when pulling c282046 on rafakato:nested-properties into 13aa291 on achingbrain:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 173d6ff on rafakato:nested-properties into 13aa291 on achingbrain:master.