Added Verify User method

Also added some async deserialising extension methods in case the user
has *LOTS* of bookmarks.
Also added the Bool JsonConverter for the User object
Added AccessToken property to interface
Used the oauth token/secret in ctor

Also renamed username param to emailAddress to make sure it's clear what
should be going in there.
Renamed methods to include Async

Sorry, funny about doing that.
Forgot to change property to a bool
Added INPC via fody
Added Params for the GetBookmarkAsync method

Also added CancellationTokens to each method
Added DebuggerDisplay to Bookmarks
Added InstaResponse to handle any API errors
DateTimeConverter for json added
Fixed converter cast exception
Removed async serialisation stuff
Merge pull request #1 from ScottIsAFool/master

Lots of great changes!