Merged selected changes from private branch 'suetanvil':

Updated .gitignore.

Fixed a typo in README.md.

Switched timer.c to clock_gettime() for higher resolution.

gd_scale.c now deals with multiple file types.

Tweaked Makefile.

build.sh now exploits multiple cores if set.

Checkout libgd master instead of 2.1.0-stable.

Enabled most modes (since that bug is fixed) in gd_resize.rb.
Switched default mode to 'bicubic'.

Added scripts for comparing image quality.

Added beginnings of a test for Pillow (i.e. fork of PIL).