Add support for courses

[dblock]

Coming from dblock/strava-ruby-client#12, Strava's route TCXs don't have activities, but have trackpoints.

{"TrainingCenterDatabase"=>
  {"Folders"=>nil,
   "Courses"=>
    {"Course"=>
      {"Name"=>"Lower Manhattan",
       "Lap"=>{"TotalTimeSeconds"=>"5255.234647358133", "DistanceMeters"=>"14914.774951561694", "Intensity"=>"Active"},
       "Track"=>
        {"Trackpoint"=>
          [{"Time"=>"2018-12-01T19:44:26+00:00", "Position"=>{"LatitudeDegrees"=>"40.7336500", "LongitudeDegrees"=>"-73.9859200"}, "AltitudeMeters"=>"9.760000000000002", "DistanceMeters"=>"0.0"},
           {"Time"=>"2018-12-01T19:44:39+00:00", "Position"=>{"LatitudeDegrees"=>"40.7333700", "LongitudeDegrees"=>"-73.9852500"}, "AltitudeMeters"=>"9.09", "DistanceMeters"=>"64.52784836429899"},
           {"Time"=>"2018-12-01T19:44:53+00:00", "Position"=>{"LatitudeDegrees"=>"40.7339800", "LongitudeDegrees"=>"-73.9848000"}, "AltitudeMeters"=>"8.860000000000001", "DistanceMeters"=>"142.3052786159879"},
           {"Time"=>"2018-12-01T19:45:07+00:00", "Position"=>{"LatitudeDegrees"=>"40.7345900", "LongitudeDegrees"=>"-73.9843500"}, "AltitudeMeters"=>"8.270000000000001", "DistanceMeters"=>"220.08269417280343"},
           {"Time"=>"2018-12-01T19:45:21+00:00", "Position"=>{"LatitudeDegrees"=>"40.7346000", "LongitudeDegrees"=>"-73.9843500"}, "AltitudeMeters"=>"8.270000000000001", "DistanceMeters"=>"220.08269417280343"},
...

Also dblock/strava-ruby-client@85b501d