Transitioning Unix installs to the XDG Base Directory Specification

Currently, Bazaar stores its configuration files and plugins under the directory ~/.bazaar on unix installs. On Windows, this is %APPDATA%/Bazaar/2.0. With the XDG Base Directory specification (http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html), many Linux and Unix platforms have tried to centralize configuration files under a specific directory referred to as $XDG_CONFIG_HOME. This has a default value of ~/.config.

Bazaar would like to be a good Unix citizen by using these standard locations for configuration files. As such, we should support that location, but not require it. Note that the following descriptions do not apply to Windows which should use their own native configuration locations. (On Windows, we currently do this by working under %APPDATA%.