Removing handlebars-helpers from dependencies

Removing the handlebars-helpers dependency and adding error
handling to give a warning if handlebars-helpers isn't found
but still allow processing to run. This is to make it so people can
use their own helpers and not be required to use the
handlebars-helpers repo.
Better warning message.

Since handlebars-helpers isn't included by default, let users
know they can still use it just by doing
`npm install handlebars-helpers`
fixing typo
Merge branch 'master' of https://github.com/assemble/assemble-handlebars
0.1.7