Fix for newer helm

Because I got error message like,

Error in post-command-hook (helm--maybe-update-keymap): (error "Nobuffer named *helm-ag*").

So I changed where user input is read.
Merge pull request #11 from syohex/fix-for-new-helm

Fix for newer helm
Bump up version and update Changes
Support using 'helm-ag' as function

It takes a directory as base directory of searching.
Merge pull request #12 from syohex/use-as-function

Support using 'helm-ag' as function
Improve 'helm-ag-pop-stack'

User can pop stack when `helm-ag` is executed in buffer which
is not related to file.
Merge pull request #13 from syohex/improve-pop-stack

Improve 'helm-ag-pop-stack'
Bump up version and update Changes