Updated comments in spell.h/.c to follow the style guide.
Refactor test_eval
vim-patch:7.4.268

Problem:  Using exists() on a funcref for a script-local function
          does not work.
Solution: Translate <SNR> to the special byte sequence.
          Add a test.

https://code.google.com/p/vim/source/detail?r=1a5ed2626b26a982e307a206572121a557adf709