Added URLImageCache and URLFileDownloader to load images.

Three main objects:
* SCVURLImageCache: An image cache where the URL is the key. The keys and last access for each image are stored in a sqlite3 db, allowing to remove old images. All files are stores in the cache directory, allowing the OS to delete the files if it requires space and avoiding to back the images up in the device's back up.
* SCVURLDownloader: A simple URL downloader. It allows to download asynchronously an URL located file. If there is more than one request for the same file, it uses a single connection. The connections are cancelled if the requester doesn't need it anymore. It handles a queue to avoid too many simultaneous connections.
* UIImage (ImageLoader): Uses the cached image if available. If not it start a URL download and stores it in the cache once it finishes. Shows an activity indicator while downloading the image.
![captura de pantalla de simulador ios 04 04 2014 10 55 15](https://cloud.githubusercontent.com/assets/1740375/2615964/d884b51e-bc00-11e3-9807-785f3474a192.png)