first try at implementing the include helper

this is the include helper as proposed in this pull request of the js
implementation: https://github.com/wycats/handlebars.js/pull/368

the test case which accesses the parent context can't be run yet because the
parser will have to be modified and I'll need some help with that once I know
if this has a chance of being merged.
Merge pull request #140 from c089/include-helper

include helper