Update README to provide more clear examples

[alissa-huskey]

I updated the README with some of my own code examples and did a little cleanup/polish.

• Replace "Quickstart" section with the code for the simplest possible use-case.
• Replace WSGI example, as I wasn't able to get the previous example working.
• Rename the previous "Quickstart" section to "How-To".
• Rename "Available funcargs" section to "Fixtures". (That seems to be the more common term. In any case, I didn't previously know that "funcargs" was another term for "fixtures", so it may be more clear to others as well.)
• Use code directives instead of literal blocks to syntax highlight python code.