Add a Dockerfile suitable for development.

This pull request includes a Dockerfile which builds an image with all requirements needed to run the tests. It doesn't install Scrapy but let's the user mount the repository path.

I made this because I found some issues while trying to install PIL in my local system and found very useful to have a image ready to run the tests without having to mess up with current system libraries or packages that are not virtualenv-friendly.

Building the image:

     docker build -t myuser/scrapydev .

Running Scrapy tests:

     docker run -t -i -v $PWD:/home/scrapy myuser/scrapydev trial scrapy