Update iOS build scripts to work with iOS 7, Xcode 5

While maintaining Xcode 4.6/10.7 compatibility.

Continuation of https://github.com/libgit2/objective-git/pull/260

- [x] Update build scripts to work with iOS 7 SDK.
- [x] Adjust build scripts to still build with Xcode 4.6 on 10.7.
- [x] Make iOS 7 deployment (+arm64 and X86_64) an option (iOS 5.0 the default).
- [x] Move all of the common build logic into one place.

Depends on https://github.com/jspahrsummers/xcconfigs/pull/16