Added a `salt.log.handlers` module which includes a queue handler.

We now use a queue handler instead of a null handler in order to show **all** log message records, at a latter stage, when we can, finally, show log messages.
Use proper pip mirrors in `integration.states.virtualenv.VirtualenvTest.test_issue_2594_non_invalidated_cache`
The salt logging mix-ins now have it's own module.
Only subclass `NewStyleClassMixIn` if on python < 2.7
Fix `unit.log_test.TestLog.test_issue_2853_regex_TypeError` by importing salt's logging from the proper module.
Renamed `salt.log.config` to `salt.log.setup`.
Allow the loader to search on different paths internally and externally.
Remove some code which should have been removed while cherry-picking.
Some log function renaming to better describe the final intention.
Extended logging handlers should be loaded from `salt.log.handlers`.
Fix `unit.log_test.TestLog.test_issue_2853_regex_TypeError` to import the proper(renamed) logging module.
`salt.loader.Loader` tags can now contain underscores.
`salt.log.handlers` is a package now.
Implement a global exception hook which logs all uncaught exceptions. Fixes #2287.
Some formatting fixes.
If we'll be looking for a `stream` attribute, first check it's presence.
Warn the user of a possible failure if not logging handlers were added.
Added a sentry logging handler using raven.
Some code cleanup, logic and variable naming improvements.

* Removed `salt.log_handlers/` left-overs.
* Allow the additional handlers to state that no handler was configured on purpose.
* Better variables naming.
Return false if no handler is to be configured.