feature/reader-discovery-mega-pr

This monstrous PR adds some new features to the Reader and fixes a few longstanding problems. Here's a brief [video demo](https://cloudup.com/ci3q3fJVM7s) of some of the new features, described below.

## Blog Detail
Displays posts in a specific blog, with an mshot of the blog as the header. To display it:

* Tap an avatar in the reader post list
* Tap an avatar in the reader post detail
* Tap an avatar in a comment beneath a post
* Tap an avatar in the list of users who like a post

Note that blog detail can only be shown for WordPress blogs - it won't appear for feeds in "Blogs I Follow."

## Redesigned Tag Activity

ReaderTagActivity has been renamed to ReaderSubsActivity and significantly redesigned.

* Switched to a ViewPager for easier movement between pages
* Recommended blogs are now shown, and integrate with the blog detail described above
* You can now follow a blog by typing its URL (with or without http://)

Note that followed blogs will be added here at a later date (waiting for some API changes).

## Backfill

In the past if you synced the reader then don't sync again for some time, with a popular tag there would often be posts missing between the posts previously retrieved and the posts just retrieved.

This PR resolves this problem to a certain degree by detecting when missing posts may exist and "backfills" them. Note that to avoid excessive bandwidth/battery consumption, there is a limit to how many posts will be backfilled.

## Issues fixed in this PR:

* Fix #249
* Fix #1183
* Fix #1297
* Fix #1299
* Fix #1313 (partial)