Printable version of reports

This can be enabled by setting 'printable = True' in a report (similar to emailable). Clicking the print button opens the report in a new window using a plain template (no menus etc) and opens the browser print dialog.

Deployed to staging for QA. Available on any of the custom reports for the `cvsulive` domain: https://staging2.commcarehq.org/a/cvsulive/reports/custom/child_protection

![report_print_button](https://f.cloud.github.com/assets/249606/714812/e196fb74-df08-11e2-9582-b63196348a79.png)