CocoaPods caching

This builds on #229.
Correct, `Podfile`, `Podfile.lock`, and `Pods` will sit in the same directory. The project or workspace however may not.

```ruby
# Podfile
platform :osx, '10.9'
workspace '../NSAttributedString+CCLFormat'
```

CocoaPods looks in the current working directory for a `Podfile`, so when you `cd` in it should work.