Better handling of null values in Mustache sections.

[airhadoken]

While working on something completely different I found bad handling of null properties in objects, like this: http://jsfiddle.net/air_hadoken/w5NPK/2/

So this is somewhat better handling of null when rendering Mustaches; it's a bit closer to how undefined would be handled.

[bmomberger-reciprocity]

This is more in line with mustache.js's null_string and null_view specs.

[andykant]

I merged portions of your pull request: The changes in Mustache.get and your test case with some minor tweaks to the template since it contained unsupported syntax ({{else}} within a base Mustache section).

I skipped the Mustache.txt changes as they seemed to primarily be for supporting the {{else}} within a base Mustache section ({{#foo.bar}} instead of the {{#if foo.bar}} helper variant). {{else}} is part of the helper extensions to Mustache and should only be used with helpers.

Currently, any template that isn't using helper enhancements should maintain compatibility with other Mustache libraries that implement the official spec.

I'm going to leave this pull request open for now in case anyone wants discuss this further.

/cc @justinbmeyer @daffl @moschel

[daffl]

Seems to be good now I think.