Allow user to override function namespace.

[iphydf]

Currently, function and type namespaces are derived from the name of the surrounding class. E.g. functions contained within class byte_vector (or after #28 perhaps class Byte_Vector) will be prefixed with byte_vector_. This has been desirable so far, because both tox and toxav follow this pattern. However, with larger namespace names, such as byte_vector, it may be better to use bv_ as the function prefix. This would require specific syntax to specify it in the apidsl file.