Added ujson, an utf8-encoded JSON exporter.

cinemateca_scrapper/cinemateca/exporters.py:
- File for adding exporters;
- Added UnicodeJsonLinesItemExporter, which adds 'ujson' exporter.

cinemateca_scrapper/cinemateca/settings.py:
- Defined FEED_EXPORTERS, a dict for defining custom exporters;
- Added 'ujson' to this dict.

cinemateca_scrapper/cinemateca/spiders/cinemateca_spider.py:
- No longer explicitly encoding strings to utf8.