Mainly adds a check to make sure the working directory is clean before
switching versions. The automatic update should not remove changes by
the user. They can do that manually if they wish.
Also restructures the function to make it a little easier to follow.
Add a spacemacs update button to the spacemacs buffer, and relabel the
package update button to read "Update Packages". This will hopefully
resolve some confusion about updating spacemacs vs. updating the
packages.
I also tweaked the switch-to-version function to make it a little more
verbose to use it for the spacemacs update button.
