0.33.0

Enhancements

• Extracted URL validation into its own module.
  Boris Bügling
  #115
  #116
• Gracefully handle unexpected source structure.
  Samuel E. Giddins
  #110
• Linter warnings and errors are now prefixed with [ATTRIBUTE_NAME].
  This ATTRIBUTE_NAME specifies which property caused the error/warning.
  Joshua Kalpin
  #122
• Add support for the specification of multiple default_subspecs.
  Kyle Fuller
  CocoaPods#2099