Added LoggerInterface::exception() interface method

For increased simplicity for logging exceptions, this patch adds an `exception()` method to the `LoggerInterface`.

This makes it much easier to log exceptions in a consistent way without loss of any existing functionality.

Currently, exceptions must be logged using verbose code:
```php
$logger->error($e->getMessage(), array("exception" => $e));
```
This is simplified to:

```php
$logger->exception($e);
    
// Optionally, the severity level can be overridden and context params can be set:
$logger->exception($e, LogLevel::INFO, array("foo" => "bar"));
```