RFC - Environment aware jobs

Created an EnvironmentAwareGearmanJobInterface

When the job implements EnvironmentAwareGearmanJobInterface, the Gearman service will inject the environment variable into the job's workload.

Example:

```
class GenerateShipmentsBatch implements GearmanJobInterface,EnvironmentAwareGearmanJobInterface
```

When this job is instantiated and passed to ```Gearman::addJob()``` the service determines that the job is an instance of ```EnvironmentAwareGearmanJobInterface``` and injects the ```environment``` value provided by configuration options