Convert deprecated write options for PHP Mongo driver >=1.5.0

This transforms the deprecated "timeout" and "wtimeout" options to their respective counterparts ("socketTimeoutMS" and "wTimeoutMS") whenever this is applicable.
Make write option conversion consistent with convertWriteConcern()

If the user supplies both the old and new write option (e.g. timeout and socketTimeoutMS), we shouldn't do anything. Only convert the deprecated option when the new option is not also present.
Merge pull request #175