Remove uri from property blacklist, rename internal uri instance variable for clarity

This is a re-submission to proper branch of #49 . It fixes #48 by removing the 'uri' property from internal property blacklist allowing the uri returned by Twilio REST to create its instance method on the eigenclass, exposing it to the user.

For clarity, the internal `@uri` instance method was also renamed to `@path`. Also, two tests were added to Twilio::REST::Recording for the `wav` and `mp3` instance methods to ensure the renaming didn't break anything.