Add the ability to configure the Mongo client via a given Mongo URI 

I let some test code but it does not compile unless you add the missing dependencies.

If you want to keep it, you need to add to the 'project.clj' file the equivalent of this 'pom.xml' content:

``` xml
<dependency>
	<groupId>junit</groupId>
	<artifactId>junit</artifactId>
	<version>4.11</version>
	<scope>test</scope>
</dependency>
<dependency>
	<groupId>de.flapdoodle.embed</groupId>
	<artifactId>de.flapdoodle.embed.mongo</artifactId>
	<version>1.34</version>
	<scope>test</scope>
</dependency>
```


Note: "de.flapdoodle.embed.mongo" dependency enable to run an embedded MongoDB server to do integration tests.

See https://github.com/flapdoodle-oss/de.flapdoodle.embed.mongo