Allow pngcairo if available

This provides nicer-looking png
Add pngcairo to pngfigure()
Uniformize coding style between my two modifications
Document use of terminal pngcairo
openmp for add/mul/div is restricted to only large arrays (arbitrary threshold set to 100k). Also fixes some c89 commenting for the new quick sort algorithms
Merge pull request #178 from 2ndforks/master

openmp for add/mul/div is restricted to only large arrays (arbitrary thr...
Merge pull request #177 from jucor/updated_pngcairo

Add pngcairo if available