Initial attempt at creating sport extension

Hi folks:

I had a great chat with Richard Wallis today regarding creating a sport(s)
extension for schema.org. He was very helpful in describing the mechanics
behind creating and testing the extension in the GitHub project. I've
incorporated his advice into a branch of our fork called 'ext-sport'.

Here's a summary of the changes:

- Created a 'sport' subdirectory under 'ext'

- Added the 'sport' extension to schema.org via the sdoapp.py file, which
registers our extension as a proper extension to the core schema.

- Created a sport.rdfa file.

- Moved the classes defined under "Sports Agent Vocabulary as of
2014-09-16" into this extension. Something had to be put into the extension
and I figured this was a good place to start. If this is not advised it is
by all means open for discussion.

- Added the appropriate links to identify the moved classes and properties
as belonging to the extension.

To view the extension, you'll need to set up a local DNS link to
"sport.localhost" in your /etc/hosts file (or Windows equivalent). You can
then run the GAE project and navigate to sport.localhost:8080 to browse the
extension.

Keep in mind this is just a first pass, questions/comments are welcome, and
once the branch is pushed we should all be able to collaborate on the
repository.

Note: At the moment I'm unable to push this branch to our fork due to a
permissions issue, once the permissions issues have been resolved I'll push
it to GitHub.

Dan