Coverage, switch from RethinkDB -> file, new API

+ Added code coverage metrics, as well as a code coverage badge
+ Put back the Travis-CI build status badge
+ Removed the "directories" in package.json
+ Instead of having 2 binaries, "flash" and "flash-import", just have
  one: "flash". "flash-import" becomes "flash import"
- RethinkDB
+ File-based flashcard storage. All of the files get put in one,
  configureable directory (config.path), which defaults to
  ~/.flashcardz
+ API is totally different. Once it stabilizes, documentation (not just
  the comments in the code) will be added.