Add example usage to docs

[andy hayden]

It would be great if the README could include some examples.

Specifically/ideally the API to extract and (if this is possible) import to numpy or a pandas DataFrame.

There seems to be interest for this both on pandas issues/mailing list and on Stack Overflow, and it would be really helpful to give a simple example, thanks!

[Jared Hobbs]

I added a toDataFrame method to the SAS7BDAT class in 813427b and updated the README to reflect the change.