implement helm-gtags-suggested-key-mappings

gtags.el has gtags-suggested-key-mapping custom variable and I love it.
So, this patch aims similar feature.

helm-gtags-suggested-key-mapping have a very few unimplemented command.
 - C-c h: 'gtags-display-browser
 - C-c g: 'gtags-find-with-grep
 - C-c v: 'gtags-find-with-idutils
 - C-c v: 'gtags-visit-rootdir
 - "\C-x4.": 'gtags-find-tag-other-window

because I didn't find similar feature in helm-gtags.el.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
Merge pull request #49 from kosaki/suggested-key-map

implement helm-gtags-suggested-key-mappings