Adds the new property "iid" to the MergeRequest Model

[berni2288]

This property has been added with Gitlab 6.2 and contains the internal merge request ID.
Afaik it is only needed when you want to generate links to Gitlab mergerequests yourself.

[m4tthumphrey]

I'm pretty sure that Issues have also adopted this ID system also, so could you add it to Issue.php too.

I aware of the 6.2 release and will get round to updating the lib ASAP.

[m4tthumphrey]

Thanks for this. It has prompted me to sort out the next version and release to go with 6.2. This and other additions can be seen in 94ac618.

Cheers!