Configargparse

- moved existing config file parsing logic into `DefaultConfigFileParser`, and added `YAMLConfigFileParser`. The config file format can now be selected by setting the `config_file_parser_class` constructor arg to one of these Parser classes.
- fixes for various bugs mostly related to config file parsing (see closed Issues). Thanks to dwfreed TimZehta wcastello and others for the pull requests.

Fixes and features:
- config file syntax is now easier to override or extend using the new constructor arg: `config_file_parser` (via merged PR 28 - thanks lahwaacz )
- support for python 2.6 (via merged PR 18 - thanks kuba )
- replace dashes with underscores in env. var names when using the auto-env-var feature (via merged PR 29 - thanks helgi )