Datasource cache example

There is a new cache implementation within datasource.

Cache instructions is completly independent so <em>datasource.get()</em> demands no changes, it is the same as in non-cache case. But cache init is required: <em>datasource.cache.initialize()</em>.

For more details see <a href="http://rawgit.com/dhilt/NGScroller/datasource-cache/src/examples/cache.html">demo</a> and sources.

<strong>NOTE:</strong> I don't understand why there are issue-4 commit changes in this pull request. I've created new branch from current hill30/NGScroller master so issue-4 files are identical in both branches... There has to be only 3 files: <em>src/examples/cache.coffee</em> (new), <em>src/examples/cache.html</em> (new), <em>src/index.html</em> (edit).