unix, windows: add uv_loop_init and uv_loop_close

These functions supersede uv_loop_new and uv_loop_delete.
uv_loop_init initialized a user allocated loop and uv_loop_close
removes all associated resources a loop uses after it has finished
execution.

uv_loop_new and uv_loop_delete are now deprecated.

@indutny, @piscisaureus: please review :-)