Support PEM file format as input format for service accounts

```
See:
https://code.google.com/p/google-api-ruby-client/#Authorization

It is not documented yet, but we plan to add support for PEM file format as 
input format instead of P12.  We should add support for it.
```

Original issue reported on code.google.com by `yan...@google.com` on 27 Mar 2013 at 5:49
