Support suppressed exceptions

This is a (partial) fix for [LOGBACK-516]. It adds support of suppressed exceptions to `ThrowableProxyConverter` and to `RootCauseFirstThrowableProxyConverter`. Also it fixes support for suppressed exceptions of suppressed exceptions (and more).

Most of code is based on support added by https://github.com/qos-ch/logback/pull/16

[LOGBACK-516]: http://jira.qos.ch/browse/LOGBACK-516