allow units to be passed for `minRam` and `minDisk`

i have wanted to do this for so damn long.  @andreaturli @grkvlt wdyt?
Why divide this by `1024`? Disk MB are normally `10^6` - see http://en.wikipedia.org/wiki/Megabyte#Definitions
Same applies for GB (10^9 for disks, and 2^30 for RAM).
Do we need different `ByteSizeStrings.parse` methods / configuration?