CocoaPods caching

This builds on #229.
Ah, I couldn't see a way to configure the location of the Podfile, but I failed to consider it being in a subdirectory.

We could attempt to auto detect it with `find . -name Podfile` or something similar, but in case of multiple Podfiles (not sure how often that might happen), this could break. In that case, I'd propose something like:

``` YAML
podfile: path/to/Podfile
```

This mirrors our `gemfile` option.

Also, just to double-check: The name of the `Podfile`, `Podfile.lock` and `Pods` files and directories don't change, and they are all in the same directory (although not necessarily the root of the repository)?

One more thing: How do I tell CocoaPods to use a different Podfile? Do I have to `cd` into the directory?