ViewModel refractoring

- Refractored ItemViewModel
- Has two constructors: PostItemViewModel(int) will pull all the data
from the server, PostItemViewModel(.....) allows you to set everything
yourself
- Has now internal logic to resolve usernames
- Added CommentModel
- Has internal logic to resolve usernames
- Changed SparklrPostControl and SparklrText to have better binding
support to ViewModels
!SampleData is currently broken
- Changed DetailsPage to take a post id as parameter, closes #66
- Added navigation for posts on profile page and search page
- Search page should now be feature complete
- Profile page should now be feature complete
- Renamed GetPostInfo to GetPostInfoAsync
- Changed some JSON objects