get_access_token.py: sending oauth_callback=oob in order to obtain PIN

Quote from twitter API documentation
> The PIN-based flow is implemented in the same exact way as Implementing Sign in with Twitter and 3-legged authorization, with the only difference being that the value for **oauth_callback must be set to oob** during the POST oauth/request_token call.
Without that, twitter will always redirect user to callback set in application settings.

*This script is very old, but pretty useful. I'll commit some other improvements later.*
*P.S. this is almost the first pull request I send to github.*