Enable libvips C++ interpolators in Travis CI

Warn about liborc 0.4.19 buffer overflows lovell#21

Version bump

Version bumps

Add usage example and further unit test for new max option

Simplify max vs crop logic

Add details of performance test task - closes lovell#12

Minor markdown layout fix

Add support for auto-scaling of width and height

Expose vips internal cache settings and status

Add support for WebP and TIFF image formats. Closes lovell#7.