Move common code out of process_robots
Client-side supplied images

Only PNG supported.
Standalone client library

Moved files around, rewritten Makefile.
Added .gitignore
Updated README, clarified licensing

Clarified licensing: my modifications to the client library have been under
LGPL, by that the original library code fell under LGPL, too (the original
client library was released as public domain).

Added license headers to individual source files. Added copyright
statements.
Fixed most compiler warnings

Two of those were real bugs.
Eliminate GNU extensions