Convert module names for plugins to strings explicitly

Previously, this line depended on the  extlib gem 
monkey patching Pathname to implement a to_str method.
Script to test performance of smartdown loading / scenario execution. 

The script prints out the time taken for to perform certain actions, 
with and without preloading turned on. It also calculates the amount 
of memory required to hold all flows in the cache.
Merge pull request #1245 from alphagov/smartdown_performance_test

Smartdown performance test
Renaming employee-parental-leave to pay-leave-for-parents
Updating meta_description
Updating references to emplyee-parental-leave in the README
rename plugin and module name
update generate_scenarios task
updating prview link in factcheck generator
updating slug and task in generate outcomes task
rename and update references in plugin test
updating references in _content.html.erb
Rename SPL factcheck generator to PayLeaveParents
Merge pull request #1247 from alphagov/change_spl_slug_and_title

Rename employee-parental-leave to pay-leave-for-parents