Added extra diagnostics

- Check if say or espeak are installed
- Check if all pip requirements were installed
OK, in this case we have different options:
1. Putting the stdout redirection in the documentation (`... > ~/jasper.log`). IMHO, the users will copy&paste the command to run the diagnostics module anyway, so it doesn't matter if it's a bit longer
2. How about adding a ``--no-logfile`` option with `argparse`?

Also, I sincerely hope we'll fix #119 soon, so `diagnose.log` might not be writable in the future (Even now: If the users feel the need to run the diagnostics module, their error might be caused by a permissions/disk space error).

We can postpone this change to a date when the issue is resolved, but we can also use a different logfile path (either with `tempfile` and `delete=false` or writing in the user's home directory (`os.path.expanduser('~/jasper.log')`). 

I prefer the first option. @crm416  @shbhrsaha I'd like to hear your opinion on this.