Add: thumbnail helper function
Extract method "resizeSlice" to reduce duplicate code.
Avoid temporaries to save some cycles.
Multiply instead of dividing
Add benchmarks to compare execution times.

Run "go test -bench LargeJpegThumb" to benchmark all functions at once.
Cache kernel weights for each row.

For each row the convolution kernel is evaluated at fixed positions around "u". By calculating these values once for each row, a huge speedup is achieved.
Remove LUT based filters.

Caching kernel weights makes using the LUT based approach obsolete. Now they seem to be even slower than their counterparts.
Remove some redundancies.
Merge branch 'master' of https://github.com/matrixik/resize into matrixik-master
Update README.md to mention resize.Thumbnail.
Merge remote-tracking branch 'nfnt/master'