Tidy up PrefixedThrowableProxyConverters

Each implementation had a bunch of code copied from the underlying
Logback implementation with the prefixing code hacked in.

Instead, lets just use regular expressions to replace the beginning of
each trace with the prefix.

Also, let's override the "Wrapped by: " text in
`RootCauseFirstThrowableConverter` and display "Causing: " instead, as
it's more intuitive.

Finally, ensure that the ThrowableProxyConverter is correctly
initialized in tests.
Remove a needless static reference

The static method has been imported, no need to explicitly name the
class.
Merge pull request #397 from nicktelford/fix/tidy-logging-prefixed-traces

Tidy up PrefixedThrowableProxyConverters