Include contents of static file version.txt as header comment in all pages

* File treemap/static/version.txt has a default header comment
* Build systems can overwrite this file with specific info about the build
* base.html template inlines it as a header comment
* The file can also be referenced from a browser as http://<site>/version.txt
Merge pull request #806 from RickMohr/version-info-header

Include contents of static file version.txt as page header comment