Build opus 1.1 for iOS 7

These changes allow one to build opus 1.1 for iOS 7. Includes the following:

* NEON optimizations enabled for armv7 and armv7s
* armv6 has been removed
* Assumes iOS 7 SDK by default
* sync.sh has been removed. Instead, manual update of git submodule is favored (FWIW, I couldn't get to sync.sh to work at all on my machine)
* changes output paths slightly; generates only two directories at the toplevel: build/ and dist/ instead of opus-* and dist-*

Some of these changes are opinionated, but I hope you find them useful.