WriteResult and related error and exception classes

Relevant issues:

 * https://jira.mongodb.org/browse/PHP-1142
 * https://jira.mongodb.org/browse/PHP-1171 (adding on to what was done in #14)
 * https://jira.mongodb.org/browse/PHP-1172 
This seems like a TODO that should be a ticket rather then inline in the code, it'll never be discovered otherwise