flamegraph.pl: allow floating point image widths
flamegraph.pl: use "blocked" (blue) node type if the function is called "sleep"
flamegraph.pl: nicer hovers
flamegraph: try to keep the node order just in case
add 'sleep' atom to stack if the process in unscheduled

- while here, try to catch exceptions of the called function
add stacks_to_flames.sh
API improvements & README updates