Make it possible to use both go and asm kernel routines
Do not process two output registers at once on vertical 2-taps resizes

The speed is the same and it will allow to resize smaller resolutions
Fallback to pure go versions and very small resolutions

Small resolutions are approximately any resize below 16x16
The smallest supported output/input res is now 2x2

Fixes #5
Check whether asm routines are bit-perfect compared to pure go
Disable some slow tests when testing.Short() is true