Podspec

Hi Héctor,

My previous pull request wasn't good enough. I've tested now and it seems to work fine.

Here are the changes I made:
- Defined macro for each component that gets enabled per subspec
- Added imports for required frameworks/libraries
- Removed warnings for missing frameworks/libraries

Pods can now be configured in the `Podfile` like this:
```ruby
pod 'ErrorKit/CoreData', :git => 'https://github.com/hectr/ErrorKit.git'
```

Thanks