Added method caching to Rabl::Engine#eval_source.
Added method caching to Rabl::Engine#eval_source.

An instance_eval on every render is costly, given that the source
never changes. So I've morphed it into a class_eval that defines a
method for each source. Even a couple hundred templates should cost
roughly nothing compared to the hundreds of thousands of requests that
hit these template each time.

With permission from my client, I can hopefully add some of my report
details, but generically I found a ~15% speed up in generic benchmarks
but less than that when I ran the templates in ratios approximating my
client's daily traffic.

nesquena/rabl,nesquena/rabl,nesquena/rabl