+ jsDelivr CDN info

Thanks to libgrabber, every time a [new release is posted](https://github.com/components/handlebars.js/releases), jsDelivr will find it & automatically upload it.  If you change your file structure, please updated the [include section](https://github.com/jsdelivr/jsdelivr/blob/master/files/handlebarsjs/update.json) or ask us to do it.

I was going to link CDNJS, but they [haven't uploaded to v2.0.0](http://cdnjs.com/libraries/handlebars.js).  Nothing against those cats; we [use their network](http://www.jsdelivr.com/network.php).
Registering undefined partial throws exception.
Fixing indentation in spec/partials.js
Implement parser for else chaining of helpers

Allows users to chain multiple helpers together using their inverse callbacks. I.e.

```
{{#if foo}}
{{else if bar}}
{{else}}
{{/if}}
```

The control flow here effectively causes the helpers to be nested. The above is actually syntactic sugar for this:

```
{{#if foo}}
{{else}}
  {{#if bar}}
  {{else}}
  {{/if}}
{{/if}}
```

Any helper may be used in this manner, the only requirement is they support normal calls and inverse calls.

Introduces a breaking change in that `{{else foo}}` may no longer be used as a root level operator. Instead `{{^foo}}` must be used.

Fixes #72.
Merge pull request #870 from max-b/master

Registering undefined partial throws exception.
Merge pull request #863 from tomByrer/patch-1

+ jsDelivr CDN info
Use toHTML vs. instanceof checks for SafeString

Allows for us to play nicely in environments such as Node that could have multiple versions of the library loaded. Also allows for implementors to provide their own behavior, provided they know what they are doing.

Fixes #886
Merge pull request #890 from wycats/to-html-safe

Use toHTML vs. instanceof checks for SafeString
Merge pull request #892 from wycats/else-if

Implement parser for else chaining of helpers
Include location information in all opcodes
Generate source maps

Allow the precompiler to generate source maps when the srcFile parameter is passed.

This refactors large chunks of the code generation pipeline, allowing metadata to be associated with code chunks as well as breaking out much of the code generation logic into a separate helper.
Add source map output support to the CLI
Provide default mapping for boilerplate code