Improve GelfLoggingFilter

GelfLoggingFilter will now added automatically by GelfLoggingBundle if
GelfConfiguration#enableRequestLog is true.
Implement GelfBootstrap and UncaughtExceptionHandlers

GelfBootstrap and UncaughtExceptionHandlers can (and should) be used to catch and log
error messages in a Dropwizard service before the GelfLoggingBundle has been properly
initialized and loaded.