Add support for bare repositories

I needed to create stats for bare repos hosted on the server. I've just changed the line in "git_stats-1.0.3/lib/git_stats/generator.rb" file in "validate_repo_path" method by adding check for "HEAD" file if ".git" folder does not exist:
```ruby
    def validate_repo_path(repo_path)
      raise ArgumentError, "#{repo_path} is not a git repository" unless (Dir.exists?("#{repo_path}/.git") || File.exists?("#{repo_path}/HEAD"))
    end
```
Probably this could be done even better, however this works for me.