Use dm-cache to speedup slower hdds

During installation give the user an option to use a part or all of his ssd as a cache for a slower hdd.
Newer kernels like 3.9 have the bm-cache functionality with which you can use some partition as a cache for another one. For example the Lenovo U310 has a 32GB ssd and a 500GB hdd. By factory default the ssd should cache the hdd. With such a mechanism the user could install elementary os on such a hybrid machine without loosing this nice idea and the possible speedup.

http://blog.kylemanna.com/linux/2013/06/30/ssd-caching-using-dmcache-tutorial/