A function to build a source from an emacs fileset

Filesets are an emacs feature for defining a grouping of files upon
which operations can be performed in bulk e.g. grep, shell commands,
etc. Filesets are mentioned here in the emacs manual:
http://www.gnu.org/software/emacs/manual/html_node/emacs/Filesets.html

You can use this function to make a helm source from a fileset. For
example, given a fileset called "Public HTML", you can construct a helm
source as follows:

(defvar my/helm-source-public-html (helm-make-source-filesets "Public HTML"))

Then my/helm-source-public-html can be added to helm-for-files-preferred-list.