Handy filters added: bytes2human and depercentage

Two filters I've made:
bytes2human - allows conversion to or from human readable for bytes (e.g. "2.5 KB" -> 2500)
depercentage - convert percentages in string to numeric value (e.g. "35.4%" -> 0.345)

I am not primarily a ruby developer... this should be rigorously code reviewed, and improved! :)