Add support for .slugignore

[CGA1123]

Closes: #9

Adds support for .slugignore files. prepare will now only copy files which do not match any entry in the .slugignore file for a given source-dir.

The logic is intended to follow logic similar to this:

      lines = File.read(slugignore_path).split
      total = lines.inject(0) do |total, line|
        line = (line.split(/#/).first || "").strip
        if line.empty?
          total
        else
          globs = if line =~ /\//
                    [File.join(build_dir, line)]
                  else
                    # 1.8.7 and 1.9.2 handle expanding ** differently,
                    # where in 1.9.2 ** doesn't match the empty case. So
                    # try empty ** explicitly
                    ["", "**"].map { |g| File.join(build_dir, g, line) }
                  end

          to_delete = Dir[*globs].uniq.map { |p| File.expand_path(p) }
          to_delete = to_delete.select { |p| p.match(/^#{build_dir}/) }
          to_delete.each { |p| FileUtils.rm_rf(p) }
          total + to_delete.size
        end
      end

Which as far as I can tell has not changes since Heroku closed-source their compiler, which can be verified by extracting the builder source from Heroku via a custom buildpack.