Use ReplSet instead of ReplSetServers in example

Use the updated name of the ReplSet class in the replicaset
documentation.

Closes #935.
Update Readme.md

Update run button to include more examples, and new features
More tuning of ha with mongos
Merge pull request #946 from Runnable/1.3-dev

Update Run button
Merge pull request #937 from evanp/issue935

Use ReplSet instead of ReplSetServers in example
Merged in missing setReadPreference #948