create model watching

  rails g model watching watcher_id:integer:index course_id:integer:index
change watchings table

  rails g migration ChangeTableWatchings
add 'watch' and 'unwatch' methods for course
watch course button shows after user signed in
display course watchers count
course watchers change with the state of course watch button
show user watched courses
Merge pull request #342 from happypeter/watcher

Watcher