SERVER-13284 Tools should use appropriate singular/plural forms of "document" and "object"

This pull request accompanies the [JIRA SERVER issue](https://jira.mongodb.org/browse/SERVER-13284) I opened a few moments ago.

The issue is trivial, but addressing it does add a little polish to MongoDB. Tools I use every day like `mongodump` and `mongorestore` for retrieving and archiving new data sometimes produce output like "imported 1 documents", "processed 1 documents", and "wrote 1 documents". I have corrected a few of these grammar mistakes by adding conditionals that choose the appropriate noun form. All tests still pass on my personal machine, a MacBook Air running Mavericks (OS X 10.9.2), but I have only tested with the "origin/master" branch from GitHub. I'm not sure whether new tests are really necessary for this sort of thing, so I haven't written any yet.