Add Handlebars Layout possibility

This is basically #1075 without any partial support. If anyone wants to add this in, he or she is welcome to do so. I might do so myself some other day, but for now, this has to suffice (**Edit**: Added). I hope this is alright as it is, and if anything should be changed before a merge, please tell me.

Oh, one more thing. You do not have to have your layouts reside in the `view/layouts` folder in this version. Anything within the view folder is possible.

**Edit**:
What this pull request does is, it adds a dependency on [express3-handlebars](https://github.com/ericf/express3-handlebars) to give the user the option to use a layout with the handlebars templating engine. The already existing layout option config/views is used, and if a layout is set and the engine is set to 'handlebars', express3-handlebars is invoked.

In addition the PR adds two new config options in config/views, namely the options partials and helpers.
The option partials allows to pass a partials directory to the handlebars module. This path should be relative to the views directory. Partials can be used like {{> partial}} as it is standard with handlebars.

The option helpers allows us, to supply global handlebars helper functions to our module.

**2nd Edit**:
Rebased to current version of branch v0.10
Also fixed this not working in case of missing layout or partials config enries