Add per-level shortcut functions

Adds per-level shortcut functions, to reduce the amount of typing needed for logging.  Especially when using fully qualified namespace paths, the log function becomes tedious. 

Not sure if you want to include this, since it does clutter up the main object a bit more.


Old style:
```php
\Analog\Analog::log($message, \Analog\Analog::DEBUG);
```
...becomes...

```php
\Analog\Analog::debug($message);
```