General email utilities.
These utilities have been tested with the Google App Engine SMTP server, and not any other SMTP servers. As a result, I can't promise that the multipart functionality will work properly outside of GAE. (Based on what I have read, multipart is one of those places where behavior seems to vary a bit by back-end.)
Normally, I would impelement utility code like this as static methods. However, it's difficult to mock static method calls, and the methods below are complicated enough that I want that option. Instead, an instance of this class should be injected into any other class that needs this functionality. It's safe to treat that instance as a singleton, but you don't have to.
@author Kenneth J. Pronovici