Added the latest manual I could find.
Added a function to retrieve the interpolation mode ID.
Merge branch '2.1.0-stable' into bicubic-fixed-perf

Added gdImageGetInterpolationMethod() to API.
Added (out-of-date) manual.
Fixed bug #86.
https://bitbucket.org/libgd/gd-libgd/issue/86/gdimagescale-segfaults-with-most
Undo previous change to make a cleaner changeset.
Fix bug #86 (segfaults on gdImageScale with most interpolation modes).