WikiaLogger: add backtrace for errors (PLATFORM-363)

@Wikia/platform-team  / @nmonterroso 

This PR adds a support for ``exception`` field in context that can be used to pass ``Exception`` class instance. This field will then be renamed to ``@exception`` and contain message and error code from the exception and  backtrace array.

An example:

```php
wfGetDB(DB_SLAVE)->query('select * from foo')
```

Data sent to Logstash:

```json
{
  "@timestamp": "2014-07-31T11:56:30.308522+00:00",
  "@message": "DBError",
  "@fields": {
    "db_name": "muppet",
    "city_id": "831"
  },
  "@exception": {
    "class": "DBQueryError",
    "message": "A database error has occurred.  Did you forget to run maintenance/update.php after upgrading?  See: https://www.mediawiki.org/wiki/Manual:Upgrading#Run_the_update_script\nQuery: select * from foo\nFunction: \nError: 1146 Table 'muppet.foo' doesn't exist (10.14.30.141)\n",
    "file": "/usr/wikia/source/app/includes/db/Database.php:941",
    "trace": [
      "/usr/wikia/source/app/includes/db/Database.php:908",
      "/usr/wikia/source/app/maintenance/eval.php(72) : eval()'d code:1",
      "/usr/wikia/source/app/maintenance/eval.php:72"
    ]
  },
  "@context": {
    "name": "muppet",
    "server": "10.14.30.141",
    "errno": 1146,
    "err": "Table 'muppet.foo' doesn't exist (10.14.30.141)"
  }
}
```
I don't think you need the comment here. It's clear what you are doing.