changes to the blog introduction text

Made some changes to the first page of the blog tutorial.

My focus is on preventing confusion.  Having trained many students to learn CakePHP I know the issues which have stumped them when setting up the blog tutorial, so I have looked to prevent any confusion.

I also moved the content about the tmp directory to below the folder structure.  When setting up CakePHP usually one is required to do this before setting up their database, and it will also tie nicely into the folder structure section above it.