Edit README. Add 'And Statement' example.

Adding a check for exposable json attributes.

Add options in increment and decrement method

Increment and decrement in a shot with upsert option

Updated use MongoID to MongoId

To use this library with mongofill

Fix belongsTo and belongToMany relations

belongsTo and belongToMany would resolve to an incorrect relation name
if it was a relation with a SQL model, due to MongoModel::belongsTo
being the caller instead of MongoModel::relation which makes foreignKey
become "belongs_to_id", the unit tests do not show this behavior because
foreignKey is set explicitly.