Added a new SIP about possible extensions of futures and promises.

[EddieJamsession]

SIP-23: Extension of Futures and Promises

[heathermiller]

We just discussed this during the Scala meeting today – we acknowledge that we've received it, we unfortunately can't take any action on it until after the release of Scala 2.11. Some time in the next few months (following the release of 2.11), the SIP Committee will meet, and this will be discussed then. Thanks for contributing!

[alexandru]

Hello, I found out about this SIP and I have some suggestions that I've written here: https://gist.github.com/alexandru/3de75c350c710161dc80

[jvican]

Since the new SIP process has been implemented, this current proposal doesn't apply anymore as a SIP. SIPs are supposed to propose changes in the main utilities that the compiler depends on. Thus, I close it. The discussion will continue in the slip repository for the moment being.