Add benchmark for serialization
Improve performance by caching entity exposures

- Prevent constant rebuilding of Entity's @exposures class instance variable,
  improving serialization performance by 25%.
Cache Entity @nested_exposures

- Was being rebuilt over and over again. Performance improvement of
  about 25%.
Avoid Kernel.dup when parsing options

- Buys another ~10% of performance gain
Create lookup for attribute names

- Avoid using the to_s, split, last, to_sym chain. Improves performance
  another ~10%.
Clean up code to satisfy RuboCop
Update changelog
Merge pull request #76 from justfalter/improve_performance

Improve performance for serialization of entities