[Engine] created MessageBuilder.AppendHtmlMessage()

this function parses a string containing any kind of html (full pages or just snippets) and adds it as smuxi-MessagePartModel to the builder.
Currently supported tags: b, strong, i, em, u, a
All other tags are assumed to have no function except change formatting through a style attribute.
Currently supported styles:
"background-color: #hex;"
"background: #hex;"
"color: #hex;"
"font-style: normal/inherit/italic;"
"font-weight: normal/inherit/bold;"
"text-decoration: underline;"

all other styles are ignored

the <img> tag does not display an image, only a hyperlink to the image.
Merge pull request #22 from oli-obk/feature/html2message