Refactor of pynag.Parsers

Long awaited refactor of pynag.Parsers. 

Summary of changes:

* Parsers.ExtraOptsParser moved to extra_opts.py
* Parsers.ParserError moved to errors.py
* Parsers.Config moved to config_parser.py
* Parsers.Livestatus moved to livestatus.py
* Parsers.MultiSite moved to multisite.py
* Parsers.RetentionDat moved to retention_dat.py
* Parsers.StatusDat moved to status_dat.py
* Parsers.SshConfig moved to ssh_config.py
* new module Utils.paths created to abstract functionality related to discovering main configuration file paths.
* new module Parsers.main.py created to create an abstract way to get values from main configuration file without import config_parsers.py.

Unit tests have been left unchanged to prove that we are maintaining backwards compatibility (this is just a refactor, minimal code changes, and as far as I know nothing should break)