TwistedOakCollapsingFutures v0.7.0 podspec file

For more information about the library, you can check the repo:

- https://github.com/Strilanc/ObjC-CollapsingFutures

or these blog posts:

- [Usage and benefits of collapsing futures](http://twistedoakstudios.com/blog/Post7149_collapsing-futures-in-objective-c)
- [Usage and benefits of cancellation tokens](http://twistedoakstudios.com/blog/Post7391_cancellation-tokens-and-collapsing-futures-for-objective-c)
- [How immortality detection works](http://twistedoakstudios.com/blog/Post7525_using-immortality-to-kill-accidental-callback-cycles)

Hopefully I did my due diligence on getting it up to snuff enough for inclusion. I've tested that the podspec passes validation, and that pointing directly at it in a podfile successfully lets me use it from other projects.