Added support for Instagram video, amends to Users endpoint, Mapper and DamianM's fix for Media.Caption property

This pull request includes a handful of bits and pieces but primarily:

 * [DamianMac's fix](https://github.com/DamianMac/InstaSharp/commit/bd0c30274ddb412eb862524c5756c74f31e6ffbc)
   to correct the type of property `Media.Caption`
 * Added support for Instagram videos, renaming `InstaSharp.Model.Image`
   to `InstaSharp.Model.MediaObject` and adding a new property
   `InstaSharp.Model.Media.Videos` of type `MediaObject`
 * Allowed the
   [`Users...Recent` endpoint](http://instagram.com/developer/endpoints/users/#get_users_media_recent)
   to specify an optional User ID
 * Overloaded the `Users...Recent` endpoint to take a `Nullable<DateTime>` where it previously took just a `DateTime`
 * Removed superfluous and redundant User ID from
   [`Users...Feed` endpoint](http://instagram.com/developer/endpoints/users/#get_users_feed)
 * Amended `Mapper.UnixTimeStampToDateTime` to use `DateTimeKind.Utc`
   instead of calling `DateTime.ToLocalTime`, per [Jon Skeet's answer](http://stackoverflow.com/a/7983514/205245) to a related StackOverflow question.

I also:

 * Removed a bunch of files that probably shouldn't be tracked (the `bin` and `obj` contents, primarily)
 * Hit "Remove and sort using directives" on the files I edited
 * Set the [`AssemblyVersion`](http://msdn.microsoft.com/en-us/library/system.reflection.assemblyversionattribute.assemblyversionattribute.aspx) to `1.0.*` rather than `1.0.0.0`