Constructor for low privilege (no clientSecret) api access.

For https://github.com/auth0/auth0.net/issues/21

Also enabled XML documentation on build, should be part of Nuget package.
I wrote them as expressions to be able to get the method name in case of test failure. See https://github.com/auth0/auth0.net/pull/27/files#diff-541ba6ab2c4407c853b6ca6787ee95f9R201