Support Log class

[Sija]

Log class (introduced in Crystal version 0.34.0) is a total rewrite of the old Logger class (now deprecated) - which raven has supported by means of automagically recording logged messages as breadcrumbs - see:

raven.cr/src/raven/integrations/kernel/logger.cr

Lines 39 to 47 in 16d04fa

	private def write(severity, datetime, progname, message)
	record_breadcrumb(
	severity,
	datetime,
	self.class.deansify(progname),
	self.class.deansify(message),
	)
	previous_def
	end

This integration needs to be reworked now in order to support the new Log class - preferably as a custom Log::Backend.

Some of the things to keep in mind:

• Usage of Logger#progname needs to be changed to use Log#source instead
• Having Raven::Logger class might prove redundant
• Having wildcard support for excluded source names (db.*) would be nice
• Raven::CrashHandler needs to be updated accordingly
• Old Logger breadcrumb integration would need to stay until Logger class is finally removed from Crystal's stdlib

Refs #64