Use recent libuv and c-ares & more loop control

# API Changes

`loop:run` method now takes an optional param `mode`, which defauls to `RUN_DEFAULT` and can be:
   - `RUN_ONCE`
   - `RUN_DEFAULT`
   - `RUN_NOWAIT`

# Implementation

- Use recent libuv and c-ares as external submodule. 
- Signal handling changed to new uv_signal_ API.

# Issues

This is no new issue, but the Signals registered are never unregistered. Thus there is a memory leek. Since the ev_signals are now part of uv_signal_, they are automatically registered. For the moment they are `uv_unref` ed to let the loop exit.