Add escaping helpers to template's render scope

This adds escaping helpers to the template's render scope. This
allows escaping HTML or URLs when needed in a template. This is
to prevent XSS attacks. These methods use the common ruby
community naming conventions of `h` for html escaping and `u`
for url escaping.

Closes #4
Merge pull request #17 from redding/jcr-escaping

Add escaping helpers to template's render scope