Renamed VEX_FUNCTION to VEX_FUNCTION_V1
Define custom functions once per kernel

This also allows to use more natural user-provided function names and
increases the overall readability of the generated kenels.
Added VEX_FUNCTION v2 macro family
Made VEX_BUILTIN_N family public
Updated 'custom functions' section of README
Merge pull request #110 from ddemidov/vex_function_v2

Vex function v2