All output from OpenRedukti should be routed via a user controllable api

[dibyendumajumdar]

Currently the C++ code can emit debug / trace / error messages directly to stderr or stdout. This is not a good idea when OpenRedukti is embedded in a scripting language; ideally all output should be routed via a user defined api, which could default to sending output to stderr, but may be replaced by the user. The main issue is how to pass a handle to the scripting language engine.