Add ci_log4net timezone configuration support

This adds `@source_tz` to log messages, currently defaulting it to `+01:00` if it's not specified. This means:

 * log shippers can specify `@source_tz` dynamically/statically;
 * raw log messages are not manipulated;
 * the timezone used for adjustments remain alongside the log message;
 * the table panel may appear to have time discrepancies (i.e. the `@timestamp` says `2013-06-21T08:00:00.012Z`, but the `@message` has `2013-06-21 09:00:00,012`).

In practice, the message:

    ERROR 2013-06-21 09:00:00,012 Margin_3 MarginRealTime MC - CA:400220534 MI:15.181891073288527743228387400 Ind:False MCP:True MCATP:False

Is parsed as:

    {
        "@fields": {
            "datetime": [
                "2013-06-21 09:00:00,012"
            ], 
            "level": [
                "ERROR"
            ], 
            "logger": [
                "MarginRealTime"
            ], 
            "message": [
                "MC - CA:400220534 MI:15.181891073288527743228387400 Ind:False MCP:True MCATP:False"
            ], 
            "thread": [
                "Margin_3"
            ]
        }, 
        "@message": "ERROR 2013-06-21 09:00:00,012 Margin_3 MarginRealTime MC - CA:400220534 MI:15.181891073288527743228387400 Ind:False MCP:True MCATP:False", 
        "@source": "stdin://ip-10-166-60-136/", 
        "@source_host": "ip-10-166-60-136", 
        "@source_path": "/", 
        "@source_tz": "+01:00", 
        "@tags": [], 
        "@timestamp": "2013-06-21T08:00:00.012Z", 
        "@type": "ci_log4net"
    }