Fixed PHP-684: add support for socketTimeoutMS

Also renamed that ambigous PHP_MONGO_DEFAULT_TIMEOUT to
PHP_MONGO_DEFAULT_WTIMEOUT

We now apply the socketTimeoutMS option if the user hasn't explicitly
modified the MongoCursor::$timeout variable
Fixed grammar in comment.
Refactor so that we only use atoi() once

Inline with the other options that we've added today.
Added missing title to test, and an extra test case.

Also rename the headers to reflect the case of the options that are tested.
Merged pull request #294