Rework the podspec

Switch to building from source to better match how CocoaPods wants to work and
reduce the required download size, and add a subspec with only the headers to
work around the issues with overlinking the libraries in unit tests.

Closes #690.
Closes #763.
Add `|| exit 1` to all of the commands in the CocoaPods setup

And update the changelog.
Merge pull request #761 from realm/tg-podspec

Rework the podspec