Cope with projects that have "Use HTTPS" option enabled

Currently, if a project has "Use HTTPS" enabled set, PivotalTracker.NET spits out an XML parsing failure, as Pivotal outputs a JSON error. I've 

* Added a detector for Pivotal errors like this that throws a more readable exception so users can debug what's wrong
* Added an optional argument to ProjectService and StoryService to make them use the HTTPS endpoint
* Added the UseHTTPS info to Project, as that's useful for users to determine when they want to use the SSL endpoint