add method addEmbeddedImageString

[michield]

On google code, this was reported as added to the 5.2.2 code, but I just tried, and it's not there on 5.2.5

http:https://code.google.com/a/apache-extras.org/p/phpmailer/issues/detail?id=119&can=1&q=AddEmbeddedImageString

probably due to the move from google to github.

[Synchro]

It was added with a different name - look at AddStringEmbeddedImage(). I guess the name was changed to make it consistent with AddStringAttachment().

[michield]

great, I've updated the phpList code to handle the change