Added streamitem (item, loader, exporter, storage and tests)

Added support to work with [streamcorpus](https://github.com/trec-kba/streamcorpus) Stream Items.

**Included:**
- ``streamitem.items.StreamItem``: Scrapy Item definition.
- ``streamitem.loaders.StreamItemLoader``: Scrapy Itemloader for ``StreamItem``.
- ``streamitem.exporters.StreamItemExporter``: Scrapy ItemExporter to .sc file.
- ``streamitem.storages.StreamItemFileFeedStorage``:: Scrapy FileFeedStorage to handle .sc files.

**Example:**

From Spider:

    def parse_page(self, response):
        loader = StreamItemLoader(item=StreamItem(), response=response)
        return loader.load_item()

Export settings:

    FEED_URI = ".scrapy/exports/streamitems.sc"
    FEED_FORMAT = "streamcorpus"
    FEED_EXPORTERS = {
        'streamcorpus': 'scrapylib.streamitem.exporters.StreamItemExporter',
    }
    FEED_STORAGES = {
        '': 'scrapylib.streamitem.storages.StreamItemFileFeedStorage',
    }