added test JSON files to project
added methods to pull tracks, mixtapes, videos data from file
cleaned up methods for reading JSON from file
removed createDataForTest:
adding tracks on updateMixtapeWithData:inRealm: to Artist
setting image of tracks to the album cover when albums are loaded
refactoring...
inMemoryRealm seems to causing issue
added helper functions and variables in DHRealmManagerTests
added additional helper method