Added a method to set/get a Handlebars instance

[OrKoN]

This also changes the API, to get the default Handlebars one has to call the using method that will return either the dependency or the previously set instance using the use method.

Maybe you like this idea. It's much more handy for me at least to use it like that.

Actually, it would be great to remove the handlebars dependency and set it explicitly:

var staticHbs = require('gulp-static-handlebars')(Handlebars);

Sorry, it's not a perfect PR, and also I could not run the tests due to some errors. Let me know what you think about changing the API like this.

[coveralls]

Coverage decreased (-38.82%) to 61.18% when pulling ded51f9 on OrKoN:master into d033646 on TakenPilot:master.

[TakenPilot]

Hmm, I think the idea has merit. Could you squash your commits to something atomic?

Are you trying to insert handlebars because of the new version 3? We could just upgrade to version 3, since the dependency badge is already complaining about that. : )

[TakenPilot]

What I mean by atomic:

• http://seesparkbox.com/foundry/atomic_commits_with_git
• http://stackoverflow.com/questions/6543913/git-commit-best-practices
• http://www.freshconsulting.com/atomic-commits/

We could also change the handlebars requirement to use >= instead of ~ to allow any handlebars version greater than the current one.

Like this: #7