Standardisation of mu_*_template helper functions

[Denperidge]

Currently documented helper functions can be found in the following commit (Denperidge-Redpencil@4db6dea) and PR #5

The following functions have been requested to not be documented; thus presumably their deletion, modification or documentation should be discussed.

• rewrite_url_header. Found in python & ruby
• validate_json_api_content_type. Found in python and ruby
• validate_resource_type. Found in python and ruby.
• update_modified. Found in python, albeit undocumented and ruby

There was also discussion on whether to keep the implicit sparql_escape, although this function is as of now still documented