Not fully initializing new StaticFile #16

The current source for Minimagick isn't completely implementing the initialisation of a new StaticFile.

https://github.com/zroger/jekyll-minimagick/blob/master/lib/jekyll-minimagick.rb#L15-L23

https://github.com/jekyll/jekyll/blob/master/lib/jekyll/static_file.rb#L29-L38

Please mirror those changes as these omissions are causing problems for other plugins, e.g. jekyll-relative-links.

Thanks.