Add MultiColumn Unique Table constraint support

[alfmatos]

Reused the Unique constraint on the Table, to provide a qui way to implement unique constraints across multiple fields. It's a rather simple approach, but gets the job done.

@Unique("a, b")
public class IntegerUniqueAnnotatedModel extends SugarRecord {

    private int a;
    private int b;

    public IntegerUniqueAnnotatedModel() {}

    public IntegerUniqueAnnotatedModel(int a, int b) {
        this.a = a;
        this.b = b;
    }

    public int getA() { return a; }
    public int getB() { return b; }
}

[sibelius]

@alfmatos thanks