Demonstrate bug in handling of compressed content

We can now add the Accept-Encoding header to the request via
IBoxConfig.AcceptEncoding. However, HttpClient isn't configured to
automatically decompress content, so gzipped responses will fail.
Enable automatic decompression of response content

- The HttpClientHandler must have 'AutomaticDecompression' enabled to
properly handle content that's compressed per IBoxConfig.AcceptEncoding.
- Add Microsoft.Net.Http package to Box.V2.Test.Integration to resolve
integration test compilation issue (see
http://davidburela.wordpress.com/2013/07/12/error-when-using-http-portable-class-library-compression/)
Resolve test failure

- Add Microsoft.Net.Http package to Box.V2.Test to resolve
MethodNotFound exception (see
http://davidburela.wordpress.com/2013/07/12/error-when-using-http-portable-class-library-compression/)
Add integration test for same.
Implement BoxUsersManager.GetEnterpriseUsersAsync