initial implementation of run command

Hi, I am eventually going to be using this Mongo library in the future and thought I would contribute some. 

This is my initial implementation of the run command, which is similar to the PyMongo implementation (http://api.mongodb.org/python/current/api/pymongo/database.html).

An example of use would be:
```julia
run_cmd(client, "test", "collstats", "collection"; scale=1024)
```
This of course will return a BSONObject with the results of the command or nothing on error
It runs the command collstats on collection in the test database with the argument scale = 1024. 

The above shows how to pass arguments to a command, you simply add a named argument to the function call.

To run a command that doesn't require a value:
```julia
run_cmd(client, "test", "isMaster")
```