Add an sklearn wrapper for the Doc2Vec model

[oxymor0n]

• TaggedDocumentTransformer transform tokenized documents to TaggedDocuments
• SklD2VModel transforms TaggedDocuments to doc2vec vectors

[oxymor0n]

oh oops sorry wasn't aware of #1462

[oxymor0n]

In any case, this is a homebrewed wrapper that we used at @pagerinc and thought you guys were missing. It's not very different than your version in #1462, but I think it still has some value since the code is more concise and the tests are (somewhat) different

[menshikh-iv]

@oxymor0n thank you for your work, we looked carefully at your code and changed several things in #1462.
Now Doc2Vec wrapper merged from #1462, for this reason I close this PR.