Enable prelude-global-helm-global-mode by default

Since Helm is another option to Ido, users who want to use it fully
should benefit from the default Prelude setup. If some users don't like,
they can always disable global Helm mode according to the instructions
in the homepage. User can enable Helm everywhere with
prelude-helm-everywhere.

This change also enables Helm version of command history in shell and
eshell. It also adds minibuffer history without overrides any key
binding in minibuffer-local-map.
Merge pull request #658 from tuhdo/master

Enable prelude-global-helm-global-mode by default