Allow a fallback to Logger when LocationAwareLogger is not in the SLF4J binding.

When multiple SLF4J bindings are present, there may be conflicts introduced with LittleProxy. This is primarily down to the use of the LocationAwareLogger. If an SLF4J binding does not provide a LocationAwareLogger, then a ClassCastException is thrown in the ProxyConnectionLogger constructor, which in turn causes the proxy to fail. This can be alleviated by providing a fallback to a standard SLF4J Logger, in cases where a LocationAwareLogger is not available.

If this pull request is deemed reasonable and accepted, I'd be happy to write tests for the fallback mechanism.