Mark the Write Options as optional in MongoWriteBatch->execute()

We inherit them from the MongoCollection in the ctor, which then
gets overwritten by the explicit Write Options argument in the ctor
.. No need to require it to be passed during execute
Start counting from 1
Add MongoWriteBatch::COMMAND_[INSERT|UPDATE|DELETE] constants
Use $mc->selectCollection(dbname(), collname(__FILE__));
Replace server error message with %s as per CR
Merge pull request #618 from bjori/BATCH-QA-write-concern-optional

MongoWriteBatch QA

* bjori/BATCH-QA-write-concern-optional:
  Replace server error message with %s as per CR
  Use $mc->selectCollection(dbname(), collname(__FILE__));
  Add MongoWriteBatch::COMMAND_[INSERT|UPDATE|DELETE] constants
  Start counting from 1
  Mark the Write Options as optional in MongoWriteBatch->execute()