Removing assemble-handlebars dependency

In an attempt to make template engines more like plugins
for Assemble, we're removing the assemble-handlebars dependency.
This will allow us to use a different "default" template engine
if we'd like. Now Assemble users will be required to do
`npm install` on their template engine of choice.
Better error message for missing engine.

Adding a friendly error message on how to install the
Assemble template engine plugin that you're looking for.
adding assemble-handlebars back into the dependencies list since I'm not good enough at git to undo the previous commit
Merge pull request #277 from doowb/master

Adding a better error message when an engine isn't found.