Let the Reader show images from private blogs. Resolves #26.

Tracks if a post's blog is private.
Posts that are on private blogs bypass photon.
The posts list will attempt to authenticate to wpcom if there is no auth cookie for the wpcom account.
Adds "image/jpg" as an acceptable mime type for images.
Gets the current topic from the model.
NUX: Create an account right away instead of waiting for blogs to be picked. Resolves #27.

If a user with no blogs signed into the app an account would not be created and the UI would be broken.
Merge branch 'release/3.7' of https://github.com/wordpress-mobile/WordPress-iOS into develop