Add support for building an iOS 8 dynamic framework

As requested.

Usage:

 1. Build with `sh build.sh ios-dynamic`
 2. Add `build/ios-dynamic/Realm.framework` to the application project
 3. Add `libc++.dylib` to the 'Link Binary With Libraries' build phase
 3. Add a 'Copy Files' build phase, set destination to 'Frameworks', and add `Realm.framework`

@alazier @jpsim @timanglade 
This is done as an Xcode build phase in the `iOS` scheme. This should be consistent, whatever choice we make. I'd prefer keeping it in `build.sh`, so we only build each architecture once, and we could remove the `Release-Combined` configuration.