Introduce AsyncAppender for logback-access: LOGBACK-827

Introduced `ch.qos.logback.access.AsyncAppender` which accepts `IAccessEvents`. 
Standard AsyncAppender from logback-classic does not support IAccessEvents and fails with exception:
~~~~
18:31:29,255 |-ERROR in ch.qos.logback.classic.AsyncAppender[access.file.async] - Appender [access.file.async] failed to append. java.lang.ClassCastException: ch.qos.logback.access.spi.AccessEvent cannot be cast to ch.qos.logback.classic.spi.ILoggingEvent
	at java.lang.ClassCastException: ch.qos.logback.access.spi.AccessEvent cannot be cast to ch.qos.logback.classic.spi.ILoggingEvent
	at 	at ch.qos.logback.classic.AsyncAppender.preprocess(AsyncAppender.java:28)
~~~~

Proposed fix for http://jira.qos.ch/browse/LOGBACK-827

Example `logback-access.xml`:
~~~~~~~xml
<?xml version='1.0' encoding='utf-8'?>
<configuration>

    <!-- always a good activate OnConsoleStatusListener -->
    <statusListener class="ch.qos.logback.core.status.OnConsoleStatusListener"/>

    <!-- See for reference: http://logback.qos.ch/access.html#configuration -->
    <appender name="access.file" class="ch.qos.logback.core.rolling.RollingFileAppender">
        <file>${catalina.base}/logs/access.log</file>
        <rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy">
            <!-- rollover daily -->
            <fileNamePattern>${catalina.base}/logs/access.%d{yyyy-MM-dd}.%i.log.tar.gz</fileNamePattern>
            <timeBasedFileNamingAndTriggeringPolicy class="ch.qos.logback.core.rolling.SizeAndTimeBasedFNATP">
                <!-- or whenever the file size reaches 100MB -->
                <maxFileSize>100KB</maxFileSize>
            </timeBasedFileNamingAndTriggeringPolicy>
        </rollingPolicy>
        <encoder>
            <pattern>combined</pattern>
        </encoder>
    </appender>

    <appender name="access.file.async" class="ch.qos.logback.access.AsyncAppender">
        <appender-ref ref="access.file"/>
    </appender>

    <appender-ref ref="access.file.async"/>
</configuration>
~~~~~~~~