Getting host alias using Environment Variable

Small change to read the hostname alias form the environment variable NCONFIG_ALIAS. If nothing is found, it will look for the HostMap.config file like it used to.

Getting the alias through an environment variable makes NConfig ready for cloud hosting like Microsoft Azure. When hosting on Azure Websites, you don't know the Hostname in advance. But you can set app settings / environment variables easily using the portal.

Also added some documentation about the alias feature in the readme.md file.