Add a command to only download new files

I'm using git-media to deploy a repository that contains hundreds of binary files. Right now the sync command (over scp) tries to verify the existence of all these files on the remote directory. Since I want this repo to be pull-only, I'm only interested in downloading any new files that have been added.

This pull request adds a new command called 'download' that skips the upload step in sync.

I also bumped the version number to 0.1.5 since this is a UI change, and also because there seems to be a discrepancy between the gemspec in this repo and the version of the gem on rubygems.org (it's 0.1.4 on rubygems).