Add hasAlpha attribute to metadata API

Provides internal sharp_image_has_alpha method

Hopefully this helps with #91
Add `background` and `flatten` APIs
Merge pull request #94 from gasi/background-flatten

Add `background` and `flatten` APIs
Allow embed with rgba background #86 #89

Small memory leak mop-up related to #94
Unit test hardening
Improve image reference handling
PNG compression level range is 0-9 #95
Use own Angle enum to wrap jcupitt/libvips@154796a
Import embedded colour profile, if present #99
Add libvips install info for Debian Jessie #100
Ensure conversion to sRGB occurs before background #99
Add support for stand-alone, pre-resize, and post-resize extraction with a topOffset, leftOffset, width, and height.