Add a _repr__ so we know what each shard does on the mapreduce dashboard.
Add a _repr__ so we know what each shard does on the mapreduce dashboard.

mikelambert/appengine-mapreduce,VirusTotal/appengine-mapreduce,GoogleCloudPlatform/appengine-mapreduce,mikelambert/appengine-mapreduce,GoogleCloudPlatform/appengine-mapreduce,VirusTotal/appengine-mapreduce,mikelambert/appengine-mapreduce,GoogleCloudPlatform/appengine-mapreduce,GoogleCloudPlatform/appengine-mapreduce,mikelambert/appengine-mapreduce,VirusTotal/appengine-mapreduce,mikelambert/appengine-mapreduce,GoogleCloudPlatform/appengine-mapreduce,VirusTotal/appengine-mapreduce,VirusTotal/appengine-mapreduce