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