add support for repairCursor command with test coverage

A new command called "repairCursor" was added to the server in 2.7.8 (see https://jira.mongodb.org/browse/SERVER-15544 for details). We need support for this in the mgo driver so that tools which involve database repair operations (like mongodump) can access it. 
It's implemented as a command which returns a cursor; the server performs a best-effort attempt to recover any data from damaged data files and return the documents it finds on the cursor - so, it could potentially return multiple copies of the same document. I've written the test to account for this behavior.

Thanks, but I'd prefer to avoid mentioning implementation details. We don't do that in any other case.

Here is a suggested version, based on yours:

// Repair returns an iterator that goes over all recovered documents in the collection,
// in a best-effort manner. This is most useful when there are damaged data files.
// Multiple copies of the same document may be returned by the iterator.