Performance improvements

Hi,

Thanks a lot for django-pipeline, it's been very helfpul for us, great project. I love the simplicity and architecture of the code base.

We've been lately experimenting a slow down of the development environment of one of our main Django projects, whose code base is quite big. After some profiling, I figured out that pipeline was behind this slow down. I've pushed a couple patches that are actually making a big difference in time load when `DEBUG = True` and template caching is disabled. This actually speeds up things more than 3x.

I'm quite sure this could actually be improved. I first developed a patch that instead of calling storage flow, simply opened the file itself, but that breaks compatibility with finders and storages. The one proposed is a little slower, but is backwards compatible. I'm open to suggestions, as i don't know finders and storages API in too much detail.

Commit messages are helpful, but let me know if you need more detail.

Thanks, cheers
Miguel