Enable reading scan stats from a pool.

lib/zfs/libzfs.rb
lib/zfs/pool.rb
	Add a Pool#scan_stats attribute
Pool properties should be Integers when possible.

lib/zfs/libzfs.rb
lib/zfs/pool.rb
test/functional/zfs_basic_test.rb
	Pool#get_property should return Integer values for numeric
	properties instead of humanized Strings.  For example, 147500
	instead of "144K".
Cleanup change 6b1934ca23fc21b018e9e59145e6cc1056c4951c

lib/zfs/libzfs.rb
lib/zfs/pool.rb
	Use Enum classes instead of case..when

test/functional/zfs_basic_test.rb
	Write functional test for scrubbing.