Add podspec for CocoaPods

CocoaPods is the primary dependency manager for iOS and Mac OS.

Including this file in the repository is not required to use libbson in CocoaPods, but it makes it simpler for developers if they fork libbson, and makes it more convenient to publish a new spec when the library releases.
Right. This file isn't getting checked into 1.1.0-rc0 though, it's getting checked into master, which is an unreleased, development version. In semver it ought to be something like 1.1.0-dev, or 1.0.3-dev, or 2.0.0-dev.

In my experience managing pods, I leave the checked-in copy of the podspec at a version number well in the future, and then edit it to the correct version, determined from semver at the time of release. Typically I push the tagged release to the server, then make the edits in my working copy, then submit the pod to the repository.

Let me do a little research on this.