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.