Determining which modules appear where is done in X resources

At startup, karuibar looks up the `karuibar.modules` array in the X
resources, and builds the (enabled) module array according to it (and in
that order). This allows for easier configuration (since configuration
now happens entirely in X resources, and no longer on compile time).

(except for which module to compile in in the first place, but I guess
that does not really fall under "configuration")

Also update readme and Makefile, remove the folder containing the
currently enabled modules (as they are now user/machine/distribution
specific), and thus rename `modules.all` back to `modules`.