Job priority and background processing

Adds options to submitJob for priority (high, low, default: none), and background. Examples:
```javascript
// High Priority
var options = {priority: "high"};
gearman.submitJob("task", data, uniq, options);

// Low Priority, Background
var options = {priority: "low", background: true};
gearman.submitJob("task", data, uniq, options);
```