Conform to the XDG base directory structure

The Freedesktop initiative has specified where applications should store their data. The spec can be found here: http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html
Ember should conform to this.

There are a series of different reasons. One is that it allows the user to better decide where data should be stored (through environment variables). It also make Ember more standards compliant, which is always a boon when working with distros.