Merge pull request #4 from nsendetzky/master

Improve compliance to mime standard and works around outlook.com behaviour
Merge pull request #5 from nsendetzky/master

Fixes missing related boundary and HTML part is always first
Merge pull request #6 from nsendetzky/master

Refactors generating the mime structure to use multipart/related only if required
Merge pull request #7 from nsendetzky/master

Extends URL view helper by server URL for creating absolute URLs
Merge pull request #8 from nsendetzky/master

Adds addHeader() method to mail message