Making helpers work like plugins.

Adding a params object to the call to a helper.register so we can
pass in grunt and assemble and they can be used from inside helpers.

New helper signature is
```js
module.exports.register = function(Handlebars, options, params) {
  Handlebars.register('awesome', function() {
    return params.assemble.options.foo;
  };
};
```
Merge pull request #376 from doowb/helper-params

Making helpers work like plugins.
updates README, changelog
Merge pull request #378 from jonschlinkert/master

updates README, changelog