Merge branch 'release/3.0.0'

* release/3.0.0: (302 commits)
  Bump the version number to 3.0.0.
  Fixed thumbnail template tag treating provided sizes as strings, not integers
  Remove source_deleted signal
  Don't send source_saved when no source
  Add failing test for source_save signal w/o source
  Test that source_saved is dispatched for new instance
  Make test less verbose
  Separate source group tests; signal counting util
  Combine source_created and source_changed
  Define NullHandler for Python <= 2.6
  Improve default cache backend handling
  pass in upscale keyword to thumbnail template tag
  Fix None checks
  Split before_access into two signals
  Reset content pointer
  Reduce calls to backend
  Store the file after we generate it
  Store cachefile strategy on file; not generator
  Add cachefile backend fallback
  Use pilkit's process_image utility
  ...