--cron2 option

This should support all possible syntaxes:

```
--cron2 key=val command
--cron2 key=val command with spaces
--cron2 command
--cron2 command with spaces
```

The only restriction is that you can't use spaces in key=val part. I'll add docs if this patch is ok.

Example:

```
$ uwsgi -s :0 -M --cron2 "unique=1 sleep 70"
[...]
Thu May 23 13:46:25 2013 - [uwsgi-cron] running "sleep 70" (pid 20732)
```