completely rename as shadowsocks-libev.

it's not conflicted with shadowsocks-python now. users can install both python and libev
ports of shaowsocks now. but they should migrate their old configuration file from
/etc/shadowsocks/config.json to /etc/shadowsocks-libev/config.json

Since this would break the name-related stuff, it'd better be included in next minor version (aka 1.6.0)