Refactor error reporting

Turn Methcla_Error into a struct that contains both an error code
and an error message. This struct is returned by value by the API
functions.

Closes #54.
Remove unneeded directory from include path
Remove unused methods
Include driver options in error message when opensl_open fails
Convert libmpg123 sound file API to new error reporting scheme
Refactor the way options are passed to the library