Improve Stream error handling lovell#88

Add support for C++11 on Mac

Small doc improvements ahead of v0.6.0

Use tile cache for interlace output

Doubles performance of Adam7 interlaced PNG output

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