API: Possibility of throttling when posting limit is reached

There is a new programming that should avoid being flooded by massive posts via API. You can define a daily, weekly and monthly posting limit. After this is reached, you can only post via web, until you fall under all defined limits. Only wall postings count. Comments don't count.

The definition is done in .htconfig.php, for example:
```
$a->config['system']['throttle_limit_day'] = 50;
$a->config['system']['throttle_limit_week'] = 200;
$a->config['system']['throttle_limit_month'] = 500;
```

This configuration would mean that a user could post 50 posts in a timeframe of 24 hours via API until he is throttled. If he does so for 4 days in a row, he won't be able to post for the next 3 days of the week. If he continues with the postings, then after 2 weeks and 2 days, the user couldn't create new postings via API for the rest of the week and an additional week. 

There are no default values. You don't have to define all value. If a value isn't defined, there is no limit check for this value. The values above are just an example.