update README

Revert "filesystem: fix bug due to filesystems spanning multiple lines"

 * Hack is no longer necessary because -P option has been added to df.

This reverts commit a7f2422.

update README with testing info

fix: sleep for maxiumum 1 sec while waiting for proc.poll()

test: add test for memory(..) (and fix memory_basic.txt)

Implement filesystem():

 * add extra variable to finish() so we can specify which filesystem
 * choose / as default filesystem to check
 * return other filesystems mounted to /dev/* in detail
 * set warn & crit default values

load(): use # of cores for load calculation

 * use /proc/cpuinfo to get # of cores
 * change default warn and crit for load to 1.0 and 2.0

fix: pass kwargs from cmd line to connect()

update README