Fixed bug with user impersonation and added List Endpoints method
Updated ServiceCatalog and UserAccess constructors and documentation
Tweak documentation for ListEndpoints
Improvements to BuildServiceCatalog

* Make the helper method static
* Services are unique on type and name, not type alone
  (e.g. compute:cloudServers and compute:cloudServersOpenStack)
Add integration test for ListEndpoints
Improve test reporting for admin calls

When admin functionality is not available to the user account under
which the integration tests are running, report inclusive results
instead of throwing an unhandled UserNotAuthorizedException.
Document the UserImpersonationResponse class
Simplify Impersonate

* Introduce local variable impersonationToken
* Avoid NullReferenceException if UserAccess is null
Merge pull request #201 from alanquillin/FixUserImpersonation

Fixed bug with user impersonation and added List Endpoints method