Fix for new helm

Don't call interactive function(such as read-string) in helm init
function.
Fix for old helm naming conventions
Improve for popping context stack
Merge pull request #4 from syohex/fix-for-new-helm

Fix for new helm
Bump up version and update Changes
Update README
Support for using `helm-ack` as function
Merge pull request #5 from syohex/fix-for-using-as-function

Support for using `helm-ack` as function
Bumpup version and update Changes