Don’t pre-calculate row heights

If done synchronously, it can block the main thread for a long time.
And as soon as the view appears, UITableView asks for all the heights, so asynchronous calculation is never done in time to be useful
Store/merge synced reader posts asynchronously
Use the right context to sync reader posts
Merge branch 'reader-performance' into reader

Conflicts:
	WordPress/Classes/ReaderPostsViewController.m
Calculate reader heights without creating cells
Cache colors/use UIButton states for performance
Adds image assets for action buttons.
Let the post title have its own UILabel on the detail page.  Lets us set the desired font styling since DTCoreText is being stubborn about it.
Merge branch 'reader' of https://github.com/wordpress-mobile/WordPress-iOS into reader
Fixes an issue where some post summaries would show html entities.
Updates post cell layout with new icons and design.
Adds OpenSans-Bold.ttf to the list of fonts available in the app.
Updates buttons on the post detail.
Follow button is moved to the post detail header.
Moves a bit of code around for organization and clarity.
Updates the font for the follow button.
Redraw the follow button after touch events since the size of the text probably changed.
Simplify image placement in the post details view.
Makes images always fill the available size. Trying this but maybe they should just be centered.