Add Interpolator built with Parslet to support nested {{}}
Use ExpressTemplates::Interpolator to interpolate {{expressions}}
during final compilation of a template (ExpressTemplate.compile())
 - This addresses a complex issue of when string interpolation takes
   place.
 - Now any {{expression}} in templates or component template fragments
   is converted into the ruby interpolation form #{expression}
   at the last possible moment, preventing much difficulty
 - However, due to the use of Parslet for expedience, this has a
   severe performance impact.  Should investigate writing our own
   optimized tokenizer to speed this up.
Comment out performance tests for now since using Parslet is slow
 - Will investigate faster solutions
Version bump v0.3.1