Add Handlebars Layout possibility

Rebase onto master of #1285

What this pull request does is, it adds a dependency on 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.