Play nice with existing helpers. AMD friendly modules format

[jimmyhchan]

The diff is hard to see but here's what we got

• Respect (but clobber) existing helper object (Issue Peaceful coexistence of user-supplied helpers #32)
  --diff Line:490/509
• AMD friendly module based on https://github.com/umdjs/umd/blob/master/jqueryPluginCommonjs.js
  The top part creates the closure and handles supporting AMD/CommonJS and falls back to the global window object. We are not setting any exports.
  I threw our current code into the bottom part and am not returning anything. Like jQuery plugins, we are only modifying the property of the global dust object so there is no need to export again.

[jimmyhchan]

this works but the core dust changes are not ready to support this.

some things i don't like:

• this is setup in it's own require wrapper but it is not really a module in that it is somewhat useless without dust.
• on the other hand, is this adequately similar to the jQuery "plugin" model

[rragan]

The original issue seems to have been closed but there is no resolution in the code. Can you elaborate what it means that the core dust changes are not ready to support this? I guess this line might be the problem:
dust.helpers = {};

Wouldn't a simple exists check before definition in core dust + the changes that were submitted as a PR here do the trick?