“ The courts act on their own discretion in affording or refusing the relief of specific performance, and as a general rule will refuse that relief where the common law remedy is adequate, where the court would be unable to superintend or enforce the execution of its judgment, where the plaintiff has himself acted inequitably, or where the enforcement of specific performance would be unreasonable. Specific performance is usually confined to executory agreements, such as a conveyance or a lease of land ”