Added client, test script, and changed readme

mongo_dart will automatically import mongo_client. Test script for
mongo_client passes if all methods return a non-null value. Updated
description in readme and replaced examples with mongo_client usage.
Added License to front page
Corrected error in examples
Update README.md
Correct error in test. Didn't close properly.
Merge pull request #57 from ericobodo/master

Promote using mongo_client as default access class. Restructure the home page with examples of using this client.