Azure Active Directory helper crate

As per PR #130.

Also:

• Reenabled CI (by Marius Seritan - PR #128).
• Added support for alternative Cosmos DB URIs, such as China, simulator and custom (PR #131).

Breaking changes

• Due to the compile-time selection of the CosmosDB URI, the Client creation syntax has been slightly changed. To fix your preexisting code, replace Client::new(..) with ClientBuilder::new(..). Variations are documented here: https://docs.rs/azure_sdk_cosmos/0.21.0/azure_sdk_cosmos/struct.ClientBuilder.html.