Enable modules in iOS build.

Speeds up compilation, and means we can avoid having to list all the system
frameworks (so I've deleted them).
Crashlytics support for official builds.

Add hooks to allow us to pass in the Crashlytics SDK location and the
API key from the command line. The SDK and API key will be held in a
separate private repository.

Should have no effect on regular builds.
Convert Xcode schemes to shared schemes, so that they are tracked in git.

It is recommended that you delete any personal schemes that have the same
name as the shared ones, for example by cleaning your git tree with:

	cd platform/ios/MuPDF.xcodeproj && git clean -X -f -d

(this will wipe out any .gitignore'd files in your tree - so be careful.
Make sure you use an uppercase -X option; the lowercase version will wipe
non-ignored files!).