Python 3 support and many more

• Full Python 3 support 👏 👍 🍻 (#106), all the thanks goes to @Preetwinder.
• canonicalize_url method removed in favor of w3lib implementation.
• The whole Request (incl. meta) is propagated to DB Worker, by means of scoring log (fixes #131)
• Generating Crc32 from hostname the same way for both platforms: Python 2 and 3.
• HBaseQueue supports delayed requests now. ‘crawl_at’ field in meta with timestamp makes request available to spiders only after moment expressed with timestamp passed. Important feature for revisiting.
• Request object is now persisted in HBaseQueue, allowing to schedule requests with specific meta, headers, body, cookies parameters.
• MESSAGE_BUS_CODEC option allowing to choose other than default message bus codec.
• Strategy worker refactoring to simplify it’s customization from subclasses.
• Fixed a bug with extracted links distribution over spider log partitions (#129).