Read config from $XDG_CONFIG_HOME/shiva/local.py

* Read config from shiva/config/local.py
* Update config with env var SHIVA_CONFIG
* If not found, look for $XDG_CONFIG_HOME/shiva/local.py file

If all fails, inform the user.
Installation using pip
Doc for configuring
Merge pull request #76 from tooxie/getenv

Added shiva.config.Configurator object