bug fix: event_test7, no need to `connect` twice.
Rewrite the core multitask, use 'generator/yield' instead of multithread. Because multithread is too slow.
Start to use libuv as a backend.
	1. implement uv_open
	2. implement write.
	3. implement close.
	4. add strerror and fs_unlink.
	5. implement read_start.
	6. use same interface for both fs and stream. unfortunately, libuv still epoll_add a regular file.
add 'lib' and '.so' when searching a module. use get_type_name() to give better error message.
use `require` to load the JS script on the command line.
add stack_trace.