Introduce a simple VSTS REST client

[olivierdagenais]

In preparation for a fair amount of interactions with VSTS, created the VstsRestClient class as a façade. A sample of its use can be found in VstsCollectionConfiguration, for testing the collection URL paired with the credentials.

Manual testing

1. Temporarily re-enabled the ping() test and ran it with the necessary parameters to connect to a VSTS account. The ping succeeded.
2. Launched a test Jenkins instance and navigated to the global configuration. For a VSTS collection URL, clicked the [Test connection] button for:
   1. Valid credentials: Success!
   2. Invalid credentials: Error: Server returned HTTP response code: 401 for URL: https://{collectionUrl}/_apis/connectiondata
   3. No credentials: Error: VSTS accounts need credentials, preferably a Personal Access Token

Mission accomplished!

[davidstaheli]

Consider using java.nio.charset.StandardCharsets.UTF_8 instead.

[olivierdagenais]

That's a good idea. StandardCharsets is Java 1.7+ and we're currently targeting Java 1.6, so I'll add an item in the backlog to upgrade to Java 1.7 (as most of Jenkins has) and to make that change at the same time.

[davidstaheli]

Looks good! See 2 minor comments.

[olivierdagenais]

Looks good! See 2 minor comments.

Thank you for the review!