Added 'Did you mean' and other improvements

WARNING: Beta version, do not use on production without extensive testing.

• Reindexing is based on events instead of class rewrites
• Solr is never empty during reindex
• Added built-in manual to Magento Configuration
• Replaced install message in admin by notification in inbox #18
• Multi word autocorrect (schema.xml update needed)
• "Did you mean" suggestions - thanks @toonvd
• Bugfix for #20: Searching on URL produces error in Solr log - thanks @toonvd
• Search in Solr for every search, to be more flexible.