Externalize writer in another library

[leodido]

Externalize writer architecture and classes.

Then require this library and extend class to add

if (!isset($doc['id'])) {
    throw new NotValidDocumentException('Document array must have an element with "id" key');
}

where it is needed.

[leogr]

I don't think that externalizing the writer is an improvement.
The writer is very specific and it could be used as standalone component already (just using zf2-sphinxsearch-tool as depedency).