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.

Please provide user-oriented documentation. The other methods are a good example for the style in which it should be worded.

Also, is there any documentation at the docs site for what the command does?