Use commas in download count

Before:

https://cloud.githubusercontent.com/assets/2/4600787/1d24d408-50dd-11e4-9174-ca840b88a08b.png

After:

https://cloud.githubusercontent.com/assets/2/4600790/56ec76be-50dd-11e4-8d96-8c7d4850c3be.png
Merge pull request #262 from atom/commas-in-download-count

Use commas in download count