Add IE8 support to nunjucks.

This is a rebase of the pull request at:

  https://github.com/jlongster/nunjucks/pull/75

In general, the fixes amount to dealing with IE8's lack of Array.indexOf(),
Object.create(), Date.now(), and string indexing via array notation.
Merge pull request #93 from toolness/ie8-fixes-rebased

Add IE8 support to nunjucks (rebased)