Download and upload images from S3

Dogestry is now able to download and upload image to/from S3 explicitly via separate commands.

dogestry push is now twice as fast, we bump up number of goroutines to 100.

Such parallelism

Much speed. So go.

List of changes:

• Use go fmt on all files.
• Use the official fsouza/go-dockerclient
• Set GOMAXPROCS to be able to use all available CPUs.
• Pulling files from S3 and pushing to S3 are parallelized using fixed pool of goroutines, currently hard-coded at 50.
• Removed the ability to set S3 prefix path. It makes debugging image name difficult.
• Logging functionality is now standarized to using fmt library.
• Added various code cleanup and test fixes.