Builds Google Protobuf 2.5.0 targeting iOS 7.

Fetches the latest release of Google Protobuf from Google, builds
using Xcode 5 and targeting iOS 7.  Builds both the protoc compiler
for use on Mac OS X and provides all protobuf runtime files needed
to build a project using protobuf generated C++ files.

The protoc compiler is left in the Pods/GoogleProtobuf/bin folder. It
can be used in a custom build rule to teach Xcode how .proto files are
turned into .h/.cc files.