Add Spyder dependency

def add(modname, features, required_version, installed_version=None,
        optional=False):
    """Add Spyder dependency"""
    global DEPENDENCIES
    for dependency in DEPENDENCIES:
        if dependency.modname == modname:
            raise ValueError("Dependency has already been registered: %s"\
                             % modname)
    DEPENDENCIES += [Dependency(modname, features, required_version,
                                installed_version, optional)]