Small wrapper class on top of HtmlEmail which encapsulates the required logic
to retrieve images that are contained in "<img src=../>" elements in the HTML
code. This is done by replacing all img-src-elements with "cid:"-entries and
embedding images in the email.
For local files the class tries to either load them via an absolute path or -
if available - use a relative path starting from a base directory. For files
that are not found locally, the implementation tries to download
the element and link it in.
The image loading is done by an instance of DataSourceResolver
which has to be provided by the caller.