[enh][mod] better query parsing ++ selecting categories with query strings

def parse_query(query):

    query_engines = []

    query_parts = query.split()

    if query_parts[0].startswith('-') and query_parts[0][1:] in engines:

        query_engines.append({'category': 'TODO', 'name': query_parts[0][1:]})

    if query_parts[0].startswith('-'):

        engine_name = query_parts[0][1:].replace('_', ' ')

        if engine_name in engines:

            query_engines.append({'category': 'none',

                                  'name': query_parts[0][1:]})

    elif query_parts[0].startswith('!'):

        category_name = query_parts[0][1:].replace('_', ' ')

        if category_name in categories:

            query_engines.extend({'category': category_name,

                                  'name': engine.name}

                                 for engine in categories[category_name])

    if len(query_engines):

        query = query.replace(query_parts[0], '', 1).strip()

    return query, query_engines

    query, selected_engines = parse_query(request_data['q'].encode('utf-8'))

    if not len(selected_engines):

    if len(selected_engines):

        selected_categories = list(set(engine['category']

                                       for engine in selected_engines))

    else:

        for pd_name, pd in request_data.items():

            if pd_name.startswith('category_'):

                category = pd_name[9:]