change name to shadowsocks-libev.

Note:

Debian/Ubuntu users may have to `sudo rm -f /etc/shadowsocks` before install
the new `shadowsocks-libev` package.

For library user, the pkg-config package is renamed to `shadowsocks-libev` but
the library name is still the `libshadowsocks` (use `-lshadowsocks` to link)

#153