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Experimental support for background commands, closes #70

This was easier than I though, but it required some changes here
and there.

The `String` object now has an internal mutex, used to keep hold of
the value of the command until it's executed.

What really changed is that:

* regular commands go through "normally"
* background commands are defined as commands that end with `&` (`$(sleep 1 &)`)
* background commands are launched in a goroutine and "locked" with the above mutex
* when the command is over, we set its output and release the mutex
* added the `cmd.wait()` function which will try to acquire the lock on the mutex, thus waiting until the command is done
* added the `.done` property on string to understand whether a command is done or not

Fun things that we can do now:

```
cmd = `sleep 10`
echo("This will be printed in 10s")

cmd = `sleep 10 &`
echo("This will be printed immediately")
sleep(9000)
cmd.done # FALSE
cmd.ok # FALSE
echo("We have been waiting 9s")
cmd.wait()
echo("And approximately another second")
cmd.done # TRUE
cmd.ok # TRUE
```

@ntwrick hat do you think?

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