Add UMD wrapper, remove separate AMD version

This PR goes with two others in the Backbone.BabySitter and Backbone.Marionette repos. I'll update this with links once all of the PRs are open.

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**Changes:**
- Remove AMD version
- Add UMD wrapper
  - update Gruntfile
  - update component.json
  - update package.json

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**UMD Details**

- AMD
  - `exports = Wreqr`
  - `Backbone.Wreqr` is defined
- CommonJS
  - `exports = Wreqr`
  - `Backbone.Wreqr` is defined
- Browser
  - `Backbone.Wreqr` is defined

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Let me know if this pull request needs anything else. Thanks!