Sphinx fulltext search engine integration

Relates to #11

Depends on #1205 and #1249

Here is a Sphinx fulltext search engine integration.
It uses ability of the Sphinx searchd daemon to work via MySQL network protocol.

The code have much in common with “yii\db”, but I have to abandon the idea of direct inheritance (instead of Connection class), since the significant differences are present. Still some classes from “yii\db” are used directly, like “Transaction” and “Expression”.

“Relation” feature is still incomplete, while I am waiting for #1205 and #1249 resolution. Still “hasOne” relations work properly both to and from Sphinx Active Record.

Suggestions and critics are welcome.