Added webhook signature verification - (#50: @gmacon)
Test PHP 5.6 and HHVM - (#51: @Nyholm)
Improved error handling - (#48: @nathanntg)
Fixed attachment handling in Message Builder - (#56 @jchamberlain)
Allow any data type in custom data - (#57: @jchamberlain)
Return non-JSON response data - (#60: @jchamberlain)
Removed legacy closing braces - (#64: @travelton)