feat(query): add transaction parameter to `findOne`

[dmk1111]

This adds Transaction support to findOne method. Now users would be able to make more requests to Datastore using one Transaction.

Closes #260

[coveralls]

Coverage remained the same at 98.335% when pulling 55b1b397fef912602bce3d540b9c750976a4bd80 on dmk1111:feat/add-transaction-find-one into 6492528254dba6c1fb22609f26808461a1655e60 on sebelga:master.

[carnun]

@dmk1111 if you can please add a unit test to https://github.com/sebelga/gstore-node/blob/master/tests/integration/model.ts then we can look at getting this into the library as soon as possible.

[dmk1111]

@carnun added. Sorry it took so much time :(

[dmk1111]

@carnun could you please tell me if there any timeframe for release with this update?