Add Podspec for the Wire compiler
Add Podspec for the Wire compiler

This podspec downloads the source at a given tag and builds the proto compiler.

To publish:

```
git tag <version>
git push origin <version>
export POD_VERSION=<version>
pod repo push WireCompiler.podspec --use-json
```

square/wire,square/wire,square/wire,square/wire,square/wire,square/wire