Added ability to pass custom SQL queries into find methods.

[adrians5j]

Example:

await SimpleEntity.count({
            sql: {
                query: "SELECT COUNT(*) FROM `SimpleEntity` WHERE age > ? OR type = ?",
                values: [20, "developer"]
            },
            groupBy: ["something"]
        });

Note that in this example groupBy will totally be ignored, because the custom sql was passed.

This behavior is supported by find, findOne and count methods.

[coveralls]

Coverage decreased (-0.01%) to 95.028% when pulling 4c097ed on entity/directSql into 7b14708 on development.